World War II lasted six bloody years from the Nazi invasion of Poland in 1939 to the final surrender of Japan on August 15, 1945. It was the deadliest and most destructive war in history, involving 30 countries, with an estimated 60-80 million people killed—most of them civilians. It was also the first time that nuclear weapons were used.

The world was polarized into two opposing blocs of totalitarian nations and their allies, the Central Powers and the Allies. The conflict was sparked by the need to control foreign territories and natural resources, which drove nations into agreements, or alliances, that guaranteed support from other states in case of war.

Militarism was rife, and the era saw a growing appetite among governments to spend enormous sums to build the largest army and navy possible. In the early twentieth century, Britain spotted an advantage in Germany’s obsession with speed, and designed a revolutionary new battleship, called HMS Dreadnought. Germany rushed to catch up and passed naval acts that raised money for new super-ships, creating a fierce arms race.

When the Lusitania, a passenger ship carrying more than 1,200 people including 128 Americans, was sunk by German submarines in May 1915, public opinion shifted away from neutrality and toward support of the Allies. Woodrow Wilson campaigned for re-election on the slogan “He Kept Us Out of War.” Women from groups such as the Mothers’ Crusade and the America First Committee picketed British ambassadors to protest British involvement in the war.