When Was World War 2? Dates, Causes, and End Explained

World War II started at different times in Europe and Asia, beginning on September 1, 1939, with Germany’s invasion of Poland (United States Holocaust Memorial Museum (official museum and education center)), while Japan had been waging war in China since 1937 (PBS (public broadcaster)). This article breaks down the key dates, causes, and players to give you a clear picture.

Duration: 6 years (1939 to 1945) ·
Start of European War: September 1, 1939 (German invasion of Poland) ·
Major Combatants: Allied Powers vs. Axis Powers ·
Estimated deaths: 70–85 million (including civilians) ·
War in the Pacific began: December 7/8, 1941 (Attack on Pearl Harbor) ·
Official end in Europe: May 8, 1945 (V-E Day)

Five key facts, one pattern: the conflict’s duration and death toll dwarfed all previous wars in history.

When did World War II start and end?

World War II in Europe: 1939 to 1945

• The war in Europe began on September 1, 1939, when Germany invaded Poland (United States Holocaust Memorial Museum).
• Britain and France declared war on Germany on September 3, 1939 (Encyclopaedia Britannica).
• Germany surrendered unconditionally on May 7–8, 1945, celebrated as V-E Day (HISTORY).

World War II in the Pacific: 1941 to 1945

• Japan attacked Pearl Harbor on December 7, 1941, bringing the United States into the war (Franklin D. Roosevelt Presidential Library).
• The war in the Pacific ended with Japan’s surrender on September 2, 1945, after atomic bombs were dropped on Hiroshima (August 6) and Nagasaki (August 9) (Encyclopaedia Britannica).

The implication: The split start and end dates reflect a truly global war that unfurled at different times across theaters, shaping the modern world order.

What started World War II and why?

The Treaty of Versailles and German resentment

• The Treaty of Versailles (1919) imposed heavy reparations, territorial losses, and a war-guilt clause on Germany (Encyclopaedia Britannica).
• This fueled deep resentment and economic hardship, which Adolf Hitler exploited to rise to power (HISTORY).

Adolf Hitler’s expansionist policies

• Hitler pursued aggressive territorial expansion: the annexation of Austria in 1938 and the occupation of Czechoslovakia in 1939 (Encyclopaedia Britannica).
• He outlined his ambitions in Mein Kampf and systematically rearmed Germany in violation of the Treaty (United States Holocaust Memorial Museum).

Invasion of Poland as the trigger

• Germany invaded Poland on September 1, 1939, after signing the Molotov-Ribbentrop Pact that ensured Soviet non-intervention (Encyclopaedia Britannica).
• Britain and France, having guaranteed Poland’s borders, declared war on Germany on September 3, 1939 (HISTORY).

The pattern: A chain of unresolved grievances, unchecked aggression, and a flawed peace treaty combined to create a flashpoint that pulled the entire world into war.

Which countries were involved in World War II?

Allied Powers

• The major Allied powers were the United States, the United Kingdom, the Soviet Union, China, and France (Encyclopaedia Britannica (summary)).
• More than 50 countries eventually joined the Allies, including Canada, Australia, India, and many occupied nations (United States Holocaust Memorial Museum).

Axis Powers

• The main Axis powers were Germany, Italy, and Japan (Encyclopaedia Britannica).
• Other Axis members included Hungary, Romania, Bulgaria, and several client states (HISTORY).

Neutral countries

• A small number of countries remained neutral, including Switzerland, Sweden, Spain, Portugal, and Turkey (Encyclopaedia Britannica).

What this means: World War II was a global war in the truest sense—almost every independent state was involved either directly or indirectly, making it the most widespread conflict in history.

How did WWII actually end?

End of the war in Europe: Germany’s surrender

• Germany surrendered unconditionally on May 7, 1945 at Reims, France, with the surrender taking effect on May 8 (V-E Day) (United States Holocaust Memorial Museum).
• Adolf Hitler had committed suicide on April 30, 1945, as Soviet forces closed in on Berlin (Encyclopaedia Britannica).

End of the war in the Pacific: atomic bombs and Japan’s surrender

• The United States dropped atomic bombs on Hiroshima on August 6, 1945, and on Nagasaki on August 9, 1945 (United States Holocaust Memorial Museum).
• Japan announced its surrender on August 15, 1945, and formally signed the Instrument of Surrender on September 2, 1945, aboard the USS Missouri — V-J Day (Encyclopaedia Britannica).

The catch: The war in Europe ended with a clear military defeat, but the Pacific war ended with a weapon that changed the nature of conflict forever—the atomic bomb, whose use remains a subject of intense debate.

Who won World War II?

Allied victory over the Axis powers

• The Allied Powers—led by the US, UK, USSR, and China—decisively defeated the Axis Powers (Encyclopaedia Britannica).
• Germany surrendered unconditionally; Japan surrendered unconditionally; Italy switched sides in 1943 after Mussolini was ousted (HISTORY).

Which country lost World War II?

• The major Axis powers—Germany, Japan, and Italy—were the losers. Their governments were dissolved, territories occupied, and leaders prosecuted for war crimes (United States Holocaust Memorial Museum).

Why this matters: The defeat of the Axis powers reshaped global politics: the United States and Soviet Union emerged as superpowers, Europe was divided, and the United Nations was created to prevent another world war.

Which country suffered the worst in WWII?

Human cost by country

• The Soviet Union suffered the highest number of casualties, with an estimated 20–27 million deaths (United States Holocaust Memorial Museum).
• Poland also experienced massive civilian and military losses, including the Holocaust, and suffered the highest proportional losses relative to its population (United States Holocaust Memorial Museum).

The Soviet Union’s losses

• Military deaths are estimated at 8.7–10.7 million, and civilian deaths at 15–20 million due to war, famine, and atrocities (United States Holocaust Memorial Museum).
• Poland lost about 6 million people, including 3 million Polish Jews, representing about 17% of its prewar population (United States Holocaust Memorial Museum).

The human toll: The war’s deadliest impact fell on the Soviet Union and Poland, with losses that still shape national memory and policy today.

Which country was blamed for starting WWII?

The Treaty of Versailles and the war guilt clause

• The Treaty of Versailles assigned sole responsibility for World War I to Germany, but it was Nazi Germany under Hitler that initiated the invasion of Poland, starting World War II in Europe (Encyclopaedia Britannica).
• Historians generally agree that while multiple factors led to the war, Nazi Germany’s aggression was the proximate cause (HISTORY).

Hitler’s role and Nazi Germany’s aggression

• Adolf Hitler’s expansionist policies and racist ideology drove Germany’s annexation of Austria, occupation of Czechoslovakia, and ultimately the invasion of Poland (United States Holocaust Memorial Museum).

The implication: The “blame” is not just historical—it shaped the post-war order, with Germany accepting responsibility and paying reparations, while Japan’s role remains a sensitive issue in East Asian diplomacy.

Timeline of World War II

• September 1, 1939 — Germany invades Poland; WWII begins. (USHMM)
• June 22, 1941 — Germany invades the Soviet Union (Operation Barbarossa). (Britannica)
• December 7, 1941 — Japan attacks Pearl Harbor; US enters the war. (FDR Library)
• June 6, 1944 — D-Day: Allied invasion of Normandy. (HISTORY)
• May 8, 1945 — V-E Day: Germany surrenders. (USHMM)
• September 2, 1945 — V-J Day: Japan surrenders, war ends. (Britannica)

The pattern: Each milestone escalated the conflict further, from a European invasion to a global conflagration.

Clarity check: What we know vs. what remains uncertain

The balance: While key dates are settled, precise human cost and some counterfactuals remain under debate.

“World War II began in Europe on September 1, 1939, with the German invasion of Poland.”

— United States Holocaust Memorial Museum (source)

“World War was an international conflict that involved virtually every part of the world. The principal belligerents were the Axis powers—Germany, Italy, and Japan—and the Allies—France, Great Britain, the United States, the Soviet Union, and, to a lesser extent, China.”

— Encyclopaedia Britannica (source)

For students of history, the clear lesson is that the combination of unresolved grievances from World War I, expansionist ambitions, and failed diplomacy led directly to the deadliest conflict in human history. The choice is not just to memorize dates, but to understand the patterns of escalation. For anyone trying to learn from the past, the implication is clear: addressing the root causes of conflict early is essential, or the cost in human lives becomes immeasurable.

Frequently asked questions

Related reading

• Flags Half Mast Today: Official Reasons by Country
• What Is a CT Scan? Uses, Preparation, and Differences from MRI