Question

Why did the United States introduce war on the side of the allies

Answer 1

In 1917, Germany, determined to win its war of attrition against the Allies, announced the resumption of unrestricted warfare in war-zone waters. Three days later, the United States broke diplomatic relations with Germany, and just hours after that the American liner Housatonic was sunk by a German U-boat.

Answer 2

There were 3 main reasons:  

Firstly, President Wilson was an idealist - he believed if Germany (an autocratic monarchy with an Absolute Ruler) won WW1, this would be "a bad thing" not just for USA, but for the whole world. The best way to stop this happening was to join the war on the side of the Allies.  

Secondly, Germany had introduced unrestricted submarine warfare from the end of January 1917. As a result, many US merchant ships were being sunk, and many US merchant seamen being killed, even though USA was neutral. Further, neutral ships sailing to and from USA were also being sunk by German submarines.All this was having an adverse effect on the US economy, and the best way to stop it was to join the war on the side of the Allies.  

Finally, in January 1917 the Germans sent the Zimmerman Telegram to the government of Mexico. This offered "substantial aid" in regaining territory lost to USA during the Mexican-American War of 1846-1848 if Mexico attacked USA. Germany had thus become a direct threat to US territory, and a declaration of war was the only possible response.

Maybe this helps U ;D.