Because they wanted to win !
The US entered the war for a variety of reasons
The US wanted to free Cuba.
WW I - Lusitania. WW II - Pearl Harbour.
The US entered the war on April 6,1917
try this website http://www.conservapedia.com/Debate:Should_the_United_States_have_entered_World_War_II%3F
1. Germans sank many of their merchant ships before the US entered the war with the their u-boats2. Germans sent a coded message to Mexico that urged Mexico to invade US that British code breakers cracked and sent to the US.
The allies entered the war in 1914; the US entered the war in 1917.
The longest war the United States ever entered was the Vietnam War.
they wanted to help Britain and France as they were all allies and powerful countries, the us had the best weapons around and after the war France and Britain would have to pay the us a great deal of money for fighting in the war
One of the main reasons the US entered World War I on the side of the allies was because the Germans had sunk the Lusitania, a passenger ship containing many Americans.
1917 smart
Un-restricted submarine warfare. If it wasn't for that, the US would not have entered the war. As it was, the US was only in it for a year anyway...it ended in 1918 (the US entered the war in 1917).