World War II, also WWII, or The Second World War, was a global military conflict that took place between 1939 and 1945. It was the largest and deadliest war in history.

Even though Japan had already been fighting China for several years, the "world" war is said to have begun on September 1, 1939 when Nazi Germany led by Adolf Hitler invaded Poland. Within two days Britain led by Sir Winston Churchhill and France declared war on Germany. Germany was not alone. Pursuant to a then secret provision of its non-agression Molotov-Ribbentrop Pact with the Soviet Union (see Russia), it was joined in the battle to conquer Poland and to divide Eastern Europe by the Soviet Union on September 17, 1939.

The Allies were initially made up of Poland, the British Empire, France, and others.

In May, 1940 Germany invaded Western Europe, taking on the British and French militaries. Six weeks later, France surrendered to Germany. Three months after that, Germany, Italy, and Japan signed a mutual defense agreement, the Tripartite Pact, and were known as the Axis Powers.

Then, nine months later, in June 1941, while still battling Britain, Germany betrayed and invaded its partner, the Soviet Union (see Russia), forcing the Soviets into the Allied camp (although they still abided by their non-aggression treaty with Japan).

In December 1941, Japan attacked the United States bringing it too into the war on the Allied side. China also joined the Allies, as eventually did most of the rest of the world.

By the beginning of 1942, the major combatants were aligned as follows: Britain, the British Commonwealth Nations, the United States and the Soviet Union were fighting Germany and Italy; and Britain, the British Commonwealth Nations, China, and the United States together with the Philippines were fighting Japan. From then through August 1945, battles raged across all of Europe, in the North Atlantic Ocean, across North Africa, throughout Southeast Asia, throughout China, across the Pacific Ocean and, by air, in Japan.

Italy surrendered in September 1943, Germany in May 1945. The surrender of Japan marked the end of the war, on September 2, 1945.

It is possible that up to 62 million people died in the war; estimates vary greatly. About 60% of all casualties were civilians, who died as a result of disease, starvation, genocide, and aerial bombing.

The former Soviet Union (see Russia) and China suffered the most casualties. Estimates place deaths in the Soviet Union at around 23 million, while China suffered about 10 million.

Poland suffered the most deaths in proportion to its population of any country, losing approximately 5.6 million out of a pre-war population of 34.8 million (16%).