> Which event is considered to have started World War II?And the answer: invasion of Poland. Photo credit: TIME, via Hulton Archive—Getty Images.World War II began in Europeon September 1, 1939, when Germany invaded Poland. Two days later, Great Britainand France responded by declaring war on Germany. Spanning a total of six years,the war was the largest and deadliest in human history, involving more than 30countries. The war finally came to an end when Germany and Japan surrendered in1945.In many ways, the second World War is considered a result of the first World Wardue to unresolved tensions and anger from that conflict. However, the GreatDepression of the 30s also led to bitterness as well as shift in publicattitudes towards dictatorship. World War I facilitated the rise in AdolfHitler's Nazi party, and while it gained support from countries like Italy andJapan, aggression was turned to the Soviet Union's practice of Communism, andultimately led to war.A few months after Japan withdrew from the failing League of Nations in 1933,Germany did the same. In 1935, Germany violated the Treaty of Versailles byintroducing mandatory military conscription and beginning rearmament. In theyears that followed, Germany continued to violate the Treaty by remilitarizingthe state and forming an alliance with Italy. Soon after, Japan and Germanyoffered support to the separatist movement in the Spanish Civil War, while theexisting government was supported by the Soviet Union. These events foreshadowedthe second World War, as both sides tested different weapons and methods ofbattle.Finally, Hitler invaded Poland in 1939. Britain and France warned Germany that,should the troops not withdraw, war was imminent. When the Nazis did nothing,Britain and France declared war on Germany, and WWII officially began.Learn more about the start of World War II below.