Austria-Hungary had control over Serbia, but Serbia did not like that fact. A group of terrorist Serbians called "The Black Hand" assassinated Archduke Ferdinand of Austria-Hungary to assert their frustration over them. This started opening up some of the secret alliances, and Austria-hungary and Serbia went to war. Germany was allied with Austria-hungary at the time, so they joined the fight and declared war on Russia, one of Serbia's allies. Then Germany declared war on France, who had England backing them up. Finally, Germany sank of the United States's ships (actually multiple), thus causing almost all major countries to go at war with one another.
