world war one started 1914 but the united states did not start til 1917 explain 2 reasons why the united states finally decied to join the war on the side of allies?

They joined the allies because Germany wanted Mexico and Japan to attack the U. S. and also because it was German U-boats that sunk the Lithuania and it killed hundreds of American civilians.

The two reasons:

Britain intercepted the Zimmerman Telegram Germany resumed unrestricted submarine warfare

Context/detail:

Public outrage in the US against the Germans swept the nation following the sinking of the British ocean liner, Lusitania - - but that happened before 1917. When a German U-boat (submarine) sank the Lusitania in May, 1915, 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 a couple years later the Germans resumed unrestricted submarine warfare, and there was also an intercepted telegram (the "Zimmerman Telegram") that showed Germany was trying to secure Mexico as an ally against the United States.

In 1917, the US declared war on Germany in response.