April 1917, Germans forced his hand, they wanted to implement submarine warfare against merchant shipping in coasts of Europe, meaning more American merchant vessels would be destroyed and US citizens would die
Woodrow Wilson consistently worked to keep US out, campaigns on in 1916 election
E.g. Peace Without Victory Speech
German submarine campaign shifts public opinion, makes it politically impossible to stay out of war
In January 1917, Germany announced that it would lift all restrictions on submarine warfare starting on February 1. This declaration meant that German U-boat commanders were suddenly authorized to sink all ships that they believed to be providing aid of any sort to the Allies. Because the primary goal was to starve Britain into surrendering, the German effort would focus largely on ships crossing the Atlantic from the United States and Canada.
The first victim of this new policy was the American cargo ship Housatonic, which a German U-boat sank on February 3, 1917. Although Wilson tried hard to keep the United States neutral, by the spring of 1917, the situation had changed significantly, and neutrality no longer seemed feasible. Germany's unrestricted submarine warfare was taking its toll, as American ships, both cargo and passenger, were sunk one after another. Finally, on April 2, Wilson appeared before Congress and requested a declaration of war. Congress responded within days, officially declaring war on Germany on April 6, 1917.
Wilson also wants to shape the terms of the peace
wanted to get collective security system centered around League of Nations