How did World War I start?

I agree with TOM6371, and also note that one of the trigger was the assassination of Austrian archduke Franz Ferdinand on June 28th 1914. The archduke was shot by Gavrilo Princip, young member of a group named Young Bosnia. Originally Gavrilo meant to shoot the archduke and Oskar Poćorek-austrohungarian general who hated Serbs, even though he was a Slav himself. Gavrilo, with other 6 assassinators, had a mission to kill the archduke and the general, but instead Gavrilo shot the archduke's wife, Sophie Chotek. After that, Austria-Hungary declared war on Serbia one month after the assassination on July 28th 1914.

Basically a dispute of land and countries dragged other countries into the mix. The US didn't have a concern in that dispute until brought into it because of the trades and treaty's with other countries.