2nd World War Summary: The Global Conflict of 1939 to 1945 Explained

Short answer

• The Second World War was a global conflict fought mainly between 1939 and 1945 by two coalitions, the Allies and the Axis powers. In Europe, it began when Germany invaded Poland on 1 September 1939, after which Britain and France declared war on Germany. Fighting later spread across Europe, North Africa, the Atlantic, and much of Asia and the Pacific. The principal Axis powers were Germany, Italy, and Japan. The principal Allies were Britain, France, the Soviet Union, the United States, and China, although the membership of each coalition changed during the war. The Allies won. Germany surrendered in May 1945, and Japan surrendered later in 1945 after the atomic bombings of Hiroshima and Nagasaki. The war killed tens of millions of people and reshaped borders, governments, and international institutions. This article summarizes the causes, the main phases, the human cost, and the aftermath, without retelling any single battle in full.

By Emma Hudson

What the name covers, and why the dates need care

People use several names for the same conflict: the Second World War, World War II, and WWII. The phrase in this article’s title, 2nd World War Summary, is the search phrase readers commonly type, but historians normally write Second World War or World War II. The names point to one connected conflict, not to unrelated wars: decisions in one theater affected another, through shared alliances, resources, and strategy.

The conventional dates, 1939 to 1945, describe the war in Europe and the period when the conflict became fully global. They do not capture every starting point. Japan and China had already been at war before 1939, and historians debate whether the wider conflict should be dated from fighting in Asia in the 1930s or from the outbreak of war in Europe in 1939. The honest approach is to use 1939 to 1945 as the standard frame, and to say plainly that fighting in China began earlier. Europe and Asia also did not stop fighting on the same day: the war in Europe ended in May 1945, while the war against Japan continued into the summer of 1945.

Why the war started: the settlement after the First World War

No single cause explains the Second World War. Historians generally point to a combination of unsettled disputes from the First World War, economic hardship, aggressive expansion by Germany, Italy, and Japan, and the failure of diplomacy to stop that expansion in time. Historians debate the weight of each factor.

The peace settlement after the First World War left grievances on several sides. Germany lost territory, faced limits on its armed forces, and was required to pay reparations. Many Germans resented the settlement, and nationalist politicians used that resentment to build support. New borders in Europe also created states with mixed populations and disputed frontiers. Encyclopaedia Britannica describes the Second World War in part as a continuation, after an uneasy 20 year break, of disputes left unsettled by the First World War. That framing is useful, but it should not be read as saying that the Second World War was inevitable. Decisions made in the 1930s still mattered.

The depression of the 1930s deepened political instability, weakening democratic governments and strengthening movements that promised national revival through force. In Germany, Adolf Hitler and the Nazi Party came to power in 1933 with an ideology built on extreme nationalism, racism, and antisemitism, and on expanding German territory in eastern Europe. In Italy, Mussolini’s Fascist government pursued expansion in Africa and the Mediterranean. In Japan, military leaders pushed for control over resources and territory in China and Southeast Asia. These separate projects converged in the late 1930s into an Axis alignment.

Appeasement, expansion, and the failure to stop aggression early

During the 1930s, Germany, Italy, and Japan each tested how far they could expand before other major powers would use force to stop them. Japan’s war in China expanded during the decade. Italy invaded Ethiopia in 1935. Germany remilitarized the Rhineland in 1936, annexed Austria in 1938, and took control of much of Czechoslovakia by 1939. Each step met protest, sanctions in some cases, and diplomacy, but not a united military response that reversed the expansion.

Britain and France pursued appeasement, making concessions in the hope of avoiding another large war. Supporters argued their countries were not ready for war; critics argued concessions encouraged further demands. A fair summary records that disagreement. What is not in serious dispute is the sequence: concessions did not stop expansion, and by 1939 Britain and France had concluded that further expansion would have to be met with war. The League of Nations, created to prevent another major war, proved unable to enforce its decisions: it had no army, its members disagreed about sanctions, and major powers were unwilling to act.

The road to war in Europe: Poland and the German Soviet pact

By the early part of 1939, Hitler had become determined to invade Poland. Poland had guarantees of French and British support if it were attacked by Germany. Before invading, Hitler sought to neutralize the risk that the Soviet Union would resist. Secret negotiations led to the German Soviet Nonaggression Pact, signed in Moscow on 23 and 24 August 1939. Britannica records that a secret protocol to that pact divided Poland between Germany and the Soviet Union, with the western part going to Germany and the eastern part to the Soviet Union. The pact shocked many observers because Nazi Germany and the Soviet Union were ideological enemies, but for both governments it served an immediate strategic purpose.

Germany invaded Poland on 1 September 1939. Britain and France declared war on Germany on 3 September 1939. The Imperial War Museums summarize the same sequence. The Soviet Union then moved into eastern Poland under the secret arrangement, so Poland was attacked from both sides and its government went into exile. Large scale land fighting in the west did not follow at once: for months the western front was relatively quiet, a period often called the Phoney War, while Poland was partitioned and the Soviet Union acted against Finland and the Baltic states.

Who fought whom: the Allies and the Axis

The simplest way to describe the coalitions is to name the principal powers, and then to note the changes. The Axis powers were Germany, Italy, and Japan. The Allies included Britain, France, the Soviet Union, the United States, and China, together with many other states, governments in exile, and colonial forces. Britannica lists the same principal belligerents, and notes that after the war France, Britain, the United States, the Soviet Union, and China became the five permanent members of the United Nations Security Council. That later fact reflects their wartime standing, not a claim that they fought as a single unified army throughout the war.

The coalitions were not fixed. Italy fought on the Axis side until 1943, when Mussolini fell and Italy changed sides, although German forces then occupied much of Italy. The Soviet Union was bound by its pact with Germany until Germany invaded it in June 1941. The United States remained formally neutral until Pearl Harbor in December 1941. France was defeated in 1940, but Free French forces continued to fight, and France was later liberated. Resistance movements operated in occupied Europe and Asia, colonial troops fought in Allied forces, governments in exile kept occupied states’ legal claims alive, and neutral states such as Sweden, Switzerland, and Ireland stayed out of the fighting while being affected by trade, blockade, and refugees.

1939 to 1941: rapid Axis expansion

The first phase of the war in Europe was marked by fast German advances. After Poland, Germany invaded Denmark and Norway in April 1940, then the Netherlands, Belgium, Luxembourg, and France in May 1940. France signed an armistice in the summer of 1940, and Britain stood alone in western Europe as the only major power still fighting Germany from a base across the Channel. The Imperial War Museums note that Britain went to war on 3 September 1939 in a mood very different from 1914, with a resigned sense that Hitler had to be stopped by force. That mood did not prevent defeat in France, but it helps explain why Britain did not seek a negotiated peace after France fell.

The Battle of Britain, fought over southern England from 10 July to 31 October 1940, according to the Imperial War Museums, prevented Germany from gaining the air control needed for an invasion of Britain. Germany then bombed British cities in the Blitz: from September 1940, over nine months, more than 43,500 civilians were killed, according to the Imperial War Museums. In 1941 Germany intervened in Yugoslavia and Greece, and in North Africa British and Commonwealth forces fought Italian and later German forces for Egypt and Libya.

1941: the war becomes truly global

The year 1941 transformed a largely European war into a world war. On 22 June 1941, Germany invaded the Soviet Union in Operation Barbarossa, breaking the German Soviet Nonaggression Pact. Britannica dates the start of the war between the Soviet Union and Germany to that invasion. The Eastern Front that followed became the largest land theater of the war in Europe, with vast armies, enormous casualties, and harsh occupation policies in Soviet territory under German control. The Soviet Union’s survival and eventual advance westward became central to the defeat of Germany, a point on which Soviet, Russian, western, and other histories agree, even when they differ about how to weigh Soviet losses, Allied aid, and Soviet leadership decisions.

On 7 December 1941, in American time, Japan attacked Pearl Harbor in Hawaii and other American, Dutch, and British positions in Asia and the Pacific. Britannica gives the date as 7 and 8 December 1941, reflecting time zone differences. The United States then entered the war, and Germany and Italy declared war on it days later. The Imperial War Museums note that by June 1942 Japanese conquests covered a vast area of Southeast Asia and the western Pacific. The entry of the Soviet Union and the United States gave the Allies the population, industry, and resources to sustain a long war.

1942 to 1943: turning points in the Pacific and in Europe

Historians often describe 1942 and 1943 as the turning point years, but the turning did not happen in a single battle or on a single day. In the Pacific, Britannica identifies the Battle of Midway, fought from 3 to 6 June 1942, as the point when the war turned against Japan. The American victory destroyed Japan’s first line carrier force and, together with the Battle of Guadalcanal, ended Japan’s ability to wage an offensive war in the Pacific. Japan remained a formidable opponent, and years of island fighting followed, but it could no longer expand as it had in late 1941 and early 1942.

In Europe, Britannica identifies the Soviet victory at Stalingrad in February 1943 as the point when the tide shifted. The defense of the city cost more than one million Soviet troops and tens of thousands of civilians their lives, according to Britannica, and the destruction of two German armies marked the beginning of the end for the German regime in the east. The battle involved months of urban fighting, encirclement, and starvation in extreme winter conditions. Other campaigns reinforced the shift: Allied victories in North Africa and Sicily in 1943 led to Mussolini’s fall, Soviet forces advanced westward after Stalingrad, and Allied navies gradually gained the upper hand against German submarines in the Atlantic, protecting supply convoys. Together, these developments reversed Axis expansion.

1944 to 1945: liberation and defeat in Europe

The final phase in Europe began with the Allied landings in Normandy on 6 June 1944, known as D-Day. The Imperial War Museums describe Operation Overlord as the start of the fight to liberate northwestern Europe from German occupation. Britannica adds that the landings opened a second front in Europe, which forced Germany to fight major land campaigns in both the east and the west at the same time. Allied forces liberated Paris in August 1944, advanced through France and the Low Countries, and entered Germany in 1945, while Soviet forces advanced from the east through Poland and into Germany.

Germany’s last major offensive in the west, in the Ardennes in the winter of 1944 to 1945, failed to reverse the Allied advance. Hitler died on 30 April 1945, according to Britannica, and the war in Europe ended on 8 May 1945. Detailed surrender and commemoration discussion belongs in the companion end of war article. Liberation did not mean the same thing everywhere: in western Europe it generally restored prewar governments, while in eastern Europe it was followed in many places by Soviet control and Communist governments. The Italian campaign also ended in 1945 after a long advance up the peninsula.

The war in Asia and the Pacific

The war against Japan should not be treated as a side story to Europe. Japan’s war in China, which began before 1939, involved large armies, occupation, atrocities against civilians, and a long Chinese resistance that tied down Japanese forces. After December 1941, Japan rapidly conquered Hong Kong, Malaya, Singapore, the Philippines, and much of Southeast Asia and the western Pacific. On 15 February 1942, the British commander at Singapore signed the largest surrender in British history, after 85,000 men were defeated by 35,000 Japanese troops, according to the Imperial War Museums.

The United States then pursued an island hopping campaign across the Pacific, capturing key islands and bypassing others. Battles at Guadalcanal, the Philippines, Iwo Jima, and Okinawa were exceptionally costly. British and Commonwealth forces fought in Burma, now Myanmar, and Chinese forces continued to resist in China. The war against Japan ended in August and September 1945 after the atomic bombings of Hiroshima and Nagasaki and the Soviet Union’s entry into the war against Japan. Britannica states that the atomic bombings knocked Japan out of the war. Historians debate the relative weight of the atomic bombings, the Soviet entry, and the blockade in Japan’s decision to surrender, and the use of atomic weapons, the only use of nuclear weapons in war, remains deeply debated.

Civilians, occupation, and the Holocaust

The Second World War was not only a war between armies. Civilians contributed to war production, endured bombing, rationing, evacuation, and displacement, and in occupied territories lived under harsh and often violent rule. Britannica stresses the total nature of the war, in which civilians not only supported the war effort but also became direct targets of aerial attack. German, Japanese, British, Soviet, Polish, Chinese, and Southeast Asian cities all suffered bombing, siege, occupation, or destruction.

Occupation policies commonly involved repression of resistance, forced labor, seizure of food and resources, and persecution of targeted groups. Resistance movements carried out sabotage, intelligence work, and escapes, often at great personal risk and with severe reprisals against civilians. Collaboration also occurred, for reasons ranging from ideology to survival, and postwar societies debated and punished it in ways that still affect national memory.

The Holocaust was the systematic, state sponsored persecution and murder of six million Jews by the Nazi regime and its collaborators, according to the United States Holocaust Memorial Museum, which states that by the end of the war nearly two out of every three European Jews had been killed. Jews were the primary victims. The Nazis also targeted Roma people, people with disabilities, Poles, Soviet prisoners of war, and others, and persecuted Communists, Jehovah’s Witnesses, gay men, and others for political or ideological reasons. The Imperial War Museums describe it in similar terms, as the systematic murder of Europe’s Jewish population by the Nazis and their collaborators. Other mass atrocities also belong in an honest summary. Japanese forces committed massacres and abuses in China and Southeast Asia. Soviet forces and the Soviet state committed deportations and killings, as Britannica notes. Allied bombing killed large numbers of civilians in Germany and Japan. These events are not equivalent in intent, scale, or organization, and historians distinguish carefully between them. The rule for this article is to attribute figures and to avoid language that celebrates killing.

How many people died: why the figures vary

Casualty figures for the Second World War vary widely, and readers should be cautious about any single number presented without a range or a source. Britannica’s main article gives a range of 40 million to 50 million deaths and calls the war the bloodiest conflict and the largest war in history. In its question and answer section, Britannica gives a wider range of estimates, from 35 million to 60 million deaths, and notes that casualty statistics are inexact. The difference between those ranges does not mean that one is wrong and the other is right. It reflects different counting methods, different start dates, missing records, and disagreements about which deaths to include, such as deaths from famine and disease linked to the war.

Two patterns are consistent across most estimates: the Soviet Union and China suffered the largest totals, according to Britannica, and civilians made up a very large share of the dead, in many estimates more than half. Britannica estimates about 5.8 million Poles died, about 20 percent of Poland’s prewar population, about 4.2 million Germans died, and about 1.97 million Japanese died. These are estimates from one major reference work, not exact counts. Where this article gives a figure, it names the source.

What changed after 1945

The United Nations was created to replace the League of Nations. Representatives of 50 countries signed its Charter in San Francisco on 26 June 1945, according to the United Nations, and the Charter came into force on 24 October 1945. The United States Department of State’s Office of the Historian gives the same sequence. The Security Council, with five permanent members, reflected the wartime coalition, and their veto power later limited the organization when they disagreed.

Europe was divided. Germany and Austria were divided into occupation zones, and Germany’s eastern territories and Poland’s borders were redrawn. The Soviet Union extended its power across eastern Europe, as Britannica notes, while western Europe rebuilt with American economic aid. The division hardened into the Cold War, a long rivalry between the United States and the Soviet Union. Decolonization accelerated after the war, as weakened colonial powers faced stronger nationalist movements in Asia and Africa. The war also enabled a Communist movement to achieve power in China, as Britannica notes, after a civil war that resumed following Japan’s defeat.

After the war, the Allies tried leading German and Japanese officials at Nuremberg and Tokyo. Those trials set precedents for individual responsibility for aggressive war and atrocities, even when defendants claimed to be following orders. They were also criticized as victors’ justice, because Allied actions such as strategic bombing were not tried in the same way. A summary should record the precedent and the criticism together.

Memory differs by country: Britain through the Blitz, Dunkirk, and D-Day; Russia and other former Soviet states as the Great Patriotic War, emphasizing Soviet sacrifice; Germany through official acknowledgment of Nazi crimes; Japan through civilian suffering and contested debates about aggression; China and Korea through memory of Japanese occupation.

A simple timeline for reference

Period	What happened in brief
1931 to 1939	Expansion by Japan, Italy, and Germany, without a united military response.
1 September 1939	Germany invaded Poland. Britain and France declared war on 3 September.
April to June 1940	Germany invaded Denmark, Norway, the Low Countries, and France.
July to October 1940	The Battle of Britain, followed by the Blitz.
22 June 1941	Germany invaded the Soviet Union.
7 December 1941	Japan attacked Pearl Harbor. The United States entered the war.
June 1942 to February 1943	Midway turned the Pacific war; Stalingrad turned the war in Europe.
1943	Allied victories in North Africa and Sicily; Italy changed sides.
6 June 1944	Allied landings in Normandy opened a second front.
May 1945	Germany surrendered, ending the war in Europe.
August to September 1945	Atomic bombings, Soviet entry, and Japan’s surrender ended the war.
1945	The United Nations Charter was signed in June and took effect in October.

Readers who want maps or end date detail should use the companion articles. This summary keeps both topics at overview level.

Frequently asked questions

When did the Second World War start and end?

The conventional dates are 1939 to 1945. In Europe, the war began with Germany’s invasion of Poland on 1 September 1939 and ended in May 1945. The war against Japan ended later in 1945. Fighting in China began before 1939, so historians debate the exact starting point.

Who were the Allies and the Axis powers?

The principal Axis powers were Germany, Italy, and Japan. The principal Allies were Britain, France, the Soviet Union, the United States, and China, plus many other states. Italy changed sides in 1943, the Soviet Union joined the Allies in 1941, and the United States entered after Pearl Harbor in December 1941.

What caused the Second World War?

Historians point to a combination of causes: unsettled disputes from the First World War, economic depression, aggressive expansion by Germany, Italy, and Japan, and the failure of the League of Nations and diplomacy to stop that expansion in time. No single cause explains the war on its own.

What were the main turning points?

Britannica identifies Midway in June 1942 as the turning point in the Pacific, and Stalingrad in February 1943 as the turning point in Europe. The Normandy landings on 6 June 1944 opened a second front in western Europe.

How many people died in the Second World War?

Estimates vary because records are incomplete and counting methods differ. Britannica gives a main range of 40 million to 50 million deaths, and a wider range of 35 million to 60 million. The Soviet Union and China suffered the largest totals, and civilians made up a very large share. Any single figure is an estimate, not an exact count.

What was the Holocaust?

The Holocaust was the systematic, state sponsored persecution and murder of six million Jews by the Nazi regime and its collaborators, according to the United States Holocaust Memorial Museum. By the end of the war, nearly two out of every three European Jews had been killed. The Nazis also targeted Roma people, people with disabilities, Poles, Soviet prisoners of war, and others.

Why did the United States enter the war?

The United States entered after Japan attacked Pearl Harbor on 7 December 1941 in American time, and attacked other Allied positions in Asia and the Pacific. Germany and Italy then declared war on the United States. Before December 1941, the United States had supplied Britain and other Allies while remaining formally neutral.

How did the Second World War change the world after 1945?

The war led to the United Nations, the division of Europe and Germany, faster decolonization, Communist victory in China, war crimes trials that set legal precedents, and a long Cold War rivalry between the United States and the Soviet Union.

Is this article the same as the article about when World War II ended?

No. This article summarizes the whole war. The companion end of war article covers end dates, surrender documents, and commemorations in detail, and the companion map article covers how the map of Europe changed phase by phase.

Sources & references

• Encyclopaedia Britannica, World War II. Summary, causes, principal belligerents, outbreak, turning points, and casualty estimates, as opened and read for this article.
• Imperial War Museums, Second World War. Overview, outbreak, Battle of Britain dates, Blitz civilian deaths, Dunkirk, D-Day, Singapore, and the war in East Asia, as opened and read for this article.
• United States Holocaust Memorial Museum, Holocaust Encyclopedia and Frequently Asked Questions about the Holocaust for Educators. Definition of the Holocaust, six million Jewish victims, nearly two out of every three European Jews killed, and other targeted groups, as opened and read for this article.
• United Nations, The Story of the Charter and UN Charter. Signing of the Charter by representatives of 50 countries in San Francisco on 26 June 1945, and entry into force on 24 October 1945, as opened and read for this article.
• United States Department of State, Office of the Historian, Milestones in the History of United States Foreign Relations: Formation of the United Nations, 1945. San Francisco conference, Charter completion, and the United Nations coming into existence on 24 October 1945, as opened and read for this article.