World War II Explained: Causes, Major Events and Lasting Global Impact

World War II was fought between 1939 and 1945 and remains the deadliest conflict in recorded history. It drew in the Axis powers—Germany, Italy and Japan—against the Allied powers, principally the United Kingdom, the Soviet Union, the United States and China. The war unfolded across Europe, the Pacific, North Africa and parts of Asia, and its outcome reshaped international politics for decades.

What caused the war

According to the United States Holocaust Memorial Museum (USHMM), Nazi leadership began planning for war from the moment Adolf Hitler's party took power in Germany in late January 1933, at a time when the country was still reeling from its defeat in the First World War. The regime pursued territorial expansion in eastern Europe under the concept of 'Lebensraum,' or living space, and USHMM states that this expansionist agenda was pursued alongside the racial persecution that led to the Holocaust.

USHMM states that Germany started the war in Europe by invading Poland on September 1, 1939. Britain and France responded by declaring war on Germany two days later, on September 3, 1939.

Major events and turning points

USHMM's encyclopedia lays out the war's key dates. Germany invaded Poland on September 1, 1939, starting the European war. By 1940 Germany had overrun much of Western Europe but failed to defeat Britain in the Battle of Britain. On June 22, 1941, Germany invaded the Soviet Union, opening the war's largest front.

The United States entered the conflict after Japan attacked the American naval base at Pearl Harbor on December 7, 1941; Washington declared war on Japan the following day, per USHMM. Allied forces landed in Normandy, France, on June 6, 1944—D-Day—opening a second front against Germany in the west.

Germany surrendered unconditionally in early May 1945, with USHMM citing separate surrender dates of May 7 in the west and May 9 in the east; May 8, 1945, is marked as Victory in Europe (V-E) Day across most of Europe. In the Pacific, Japan agreed in principle to unconditional surrender on August 14, 1945, and formally surrendered on September 2, 1945, according to USHMM. This followed the atomic bombings of Hiroshima on August 6, 1945, and Nagasaki on August 9, 1945, and the Soviet Union's declaration of war on Japan on August 8, 1945.

The Holocaust

Alongside the military war, Nazi Germany carried out the systematic murder of six million European Jews, a campaign USHMM defines as the Holocaust, occurring between 1933 and 1945. USHMM states that killings were carried out mainly through mass shootings—which killed up to two million people, often at mass graves or via mobile gas vans—and through five stationary killing centers in occupied Poland (Chelmno, Belzec, Sobibor, Treblinka and Auschwitz-Birkenau), where nearly 2.7 million Jews were murdered using poison gas.

The human cost

Casualty estimates vary by source. The National WWII Museum in New Orleans puts worldwide battle deaths at roughly 15 million, battle wounded at about 25 million, and civilian deaths at about 45 million, while cautioning that "worldwide casualty estimates vary widely." The museum notes Chinese civilian deaths alone "might well be more than 50,000,000" by some counts, underscoring how uncertain the global totals remain.

Global impact and aftermath

The war's end reshaped the international system. Representatives of 50 countries met at the United Nations Conference on International Organization in San Francisco from April 25 to June 26, 1945, where they drafted and signed the UN Charter, according to the United Nations' own history page. The Charter was ratified by China, France, the Soviet Union, the United Kingdom and the United States along with a majority of other signatories, and the UN officially came into being on October 24, 1945. The UN's history page states that at the time, "nations were in ruins, and the world wanted peace," and that the organization was created to maintain international peace and security, provide humanitarian assistance, protect human rights and uphold international law.

The war also accelerated the decline of European colonial empires and set the stage for the Cold War rivalry between the United States and the Soviet Union, which had fought as allies against Germany but emerged as the two dominant postwar powers.