America Declares War!
At the point when Germany announced war on Britain, we suspected that we should get involved and try to help, considering we were allies with Britain. Even though a portion of the nation would not like to join the war, we had already gave them about 2.3 billion dollars in bolster, so it would be ridiculous to not join in and help them.
The Zimmerman Note
The British blocked a telegram that said the Germans were going to have Mexico attack the United States if the US joined the war. Germany, likewise, guaranteed Mexico that on the off chance that they assaulted the United States, they would offer them some assistance with gaining back old territory from the United States.
U.S Merchant Ships
Hmm... Why would the United States be angry about this? Four U.S Merchant ships with copious citizens on board were sunk by Germany for absolutely no reason whatsoever? I think this one is self explanatory, the United States was clearly enraged.
When the Russian Czar was removed from war, the United States joined in, because they thought that they needed to participate in the war solely for democracy. The war then became a Democracy vs. Autocracy, which gave the United States an excuse to join the war. Democracy!