Answer:
<h2>The United States entered the conflict, siding with the Allied Powers against Germany.</h2>
Context/detail:
Public outrage in the US against the Germans swept the nation following the sinking of the British ocean liner, Lusitania, which had happened in May, 1915. When a German U-boat (submarine) sank the Lusitania, over 1,000 persons were killed, including more than 100 Americans. The passenger liner was targeted by the Germans because they suspected weapons were being shipped to Britain in the cargo hold of the ship.
Germany managed to stave off American entry into the war at the time by pledging to stop submarine attacks. But two years, the Germans resumed such attacks. This inflamed public opinion against Germany once again and poised America to go to war.
In addition, British intellegence intercepted and decoded a telegram (known as the "Zimmermann Telegram") that showed Germany was trying to secure Mexico as an ally against the United States. The revelation of this plan inflamed feelings in the United States. The telegram was sent in January, 1917. In March, 1917, German Foreign Secretary Arthur Zimmermann admitted the telegram was, in fact, sent by his office.
In April, 1917, the US declared war on Germany in response to the German's submarine warfare and the Zimmermann Telegram.
