On this day in 1917, President Woodrow Wilson asked Congress to declare war against Germany, after the European nation broke its word that it wouldn't engage in unrestricted submarine warfare in the North Atlantic Ocean and Mediterranean Sea. Wilson also cited Germany's attempt to form an alliance with Mexico to oppose the U.S., if the latter decided to enter the war. Germany knew it was playing with fire; around a year earlier, it sank an unarmed French boat, and Wilson warned that he would cut off diplomatic relations unless it stopped attacking passenger ships.
A year later, that arrangement collapsed when the Germans flip-flopped because of the Americans' support for the Allies. The U.S. was sending funds and munitions to the Allies, and participated in a blockade of Germany. Military officials convinced Germany's leadership to break their word, as the U.S. wasn't, in their view, a neutral player. One guy in the room, Chancellor Theobald von Bethmann-Hollweg, thought it was a horrible idea that would lead to his nation's defeat. This was prescient; the U.S. would be one of the most effective forces in World War I.
In February 1917, after receiving news that Germany would start attacking ships again, Wilson followed through on his promise and severed diplomatic ties. Germany didn't care for that and started sinking U.S. boats. Between that and the Germans' backdoor negotiations with Mexico, the American public was ready to declare war.
Seeing no other option, Wilson requested a declaration of war during a joint session of Congress. A handful of days later, Congress did so. After ramping up its military machine, the U.S. was able to send millions of soldiers to Europe and mobilize its society in support of the war effort. With the economic might of the U.S. on its side, the Allies had the momentum to finally end the war.
You can read more about the U.S. decision to enter WWI at History.com.
