Why did the US get involved in World War 1?

Answer 1

the single most important reason the U.S. got involved in the war was because they felt that their position as a neutral country had been violated because a German U-boat had sunk the Lusitania, an American passenger ship.

AnswerThe Lusitania was not an American passenger ship, but a British one, though American passengers were aboard, and some of them died. Its sinking led to a furious diplomatic incident, but never came even close to bringing the US into the war. That came only two years later, when Germany had indeed started sinking US (and other neutral) merchantmen.

Just in case this wasn't enough, the German Foreign Office sought an alliance with Mexico, ofering them the return of Texas, New Mexico and Arizona in return for support should the US enter the war. This message (the so-called "Zimmermann Telegram" was intercepted and decoded, and pulled the rug out from under those Americans still favouring neutrality.