It was the Japanese sneak attack on the American base at Pearl Harbor on Dec. 7, 1941.
Germany
This is because the U.S.A. is finally joining the modern world.
German U-boats sunk the the Ship the Lusitania. It was carrying American civilians. America was close to joining the war, this just pushed America into it faster and on the side of England.
Serbia
The attack on Pearl Harbour, by the Japanese, directly brought the USA into the second World War. America declared war on Japan, and then Germany declared war on the USA, some days later.
The Russians pushed from the East. The Allies pushed from the North and South.
The pros for the United States joining World War 1 include the reputation of a strong country,they had many alliances, and political gain on the world system. The cons of them joining the war include death, great casualties, and the loss of military strength.
Its World War 1 u idiot...
The Holocaust.
The Allies won.
America declaring war on Nazi Germany and joining the Second World War in Europe.
Adolf Hitler.