Why Did the First World War Happen?

World War started when Serbia assassinated the Archduke Francis Ferdinand. Austria Hungary was unsure about declaring war on Serbia until German promised they would fight with Austria. They Serbia asked Russia to fight with them and they said yes. But, Russia couldn't do it alone so they asked France who also said yes. As German was going to attack France they when into Berlin which caused Britain ( a neutral country at first) to join Serbia in the war. It wasnt until Germans attacked a ship with Americans did the US join. With the US in the war we were able to win the battle against Germany and Austria Hungary.