What event was the final straw that forced the U. S. to enter World War I?

The final straw came when Great Britain shared the intercepted Zimmermann Telegram with the United States, revealing that Germany had promised American territory to Mexico in return for attacking the U.S. if it entered the war.

Explanation:

The British informed Wilson that the Entente was about to win the war, and warned him that he (Wilson, personally) would not have a seat at the peace conference unless the US entered the war. Since Wilson desperately wanted to be in on that conference, as that way he could dazzle everyone with his brilliance and convince them all to agree to set up his utopian society, he determined to con Congress into declaring war on Germany. Submarine warefare (which was fundamentally no different from the mainly-British naval blockade) and the lunatic Zimmerman telegram were just Wilson's excuses.

The US was not “forced" to enter WW1 in any way.

Once the US already had declared war the British gave Wilson the bad news: the Entente was actually about to lose, due to going broke. The US ended up financing the rest of the Entente side of the war, and the main Entente nations (Britain and France) defaulted on their war debts in 1930 and then laughed at “Uncle Shylock.”

Wilson was shocked that no one at the peace conference paid any attention to his nonsense. They did establish a “League of Nations,” but only as a way of enforcing colonialism.

The US entered WW1 entirely due to Wilson's delusions.

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