1: I think the answer is that Archduke Franz Ferdinand was assassinated in Austria-Hungary, therefore starting World War 1.
2: There were two reasons. The Germans sunk the Lusitania Ship in 1915 which was carrying submarine weapons and civilians and Germany sent a telegram to Mexico (Zimmermann Telegram) telling them to attack America because Germany feared that America was gonna join the war. The British caught the message, translated it, and showed it to the Americans and Americans were then shipped out to Europe.
