The proximate rationale was Germany's resumption of unrestricted U-boat warfare. But of course Wilson -- in a very Wilsonian way -- spoke of higher causes, especially the defense of democracy. It was in this speech that he used the term of phrase "the world must be made safe for democracy."
"The present German submarine warfare against commerce is a warfare against mankind," he asserted. "It is a war against all nations. American ships have been sunk, American lives taken in ways which it has stirred us very deeply to learn of; but the ships and people of other neutral and friendly nations have been sunk and overwhelmed in the waters in the same way. There has been no discrimination. The challenge is to all mankind.
"Each nation must decide for itself how it will meet it. The choice we make for ourselves must be made with a moderation of counsel and a temperateness of judgment befitting our character and our motives as a nation. We must put excited feeling away. Our motive will not be revenge or the victorious assertion of the physical might of the nation, but only the vindication of right, of human right, of which we are only a single champion." (The entire speech is here.)
Not everyone was behind Wilson at this point. Maverick Sen. Robert La Follette Sr. of Wisconsin (also pictured) was having none of it. He didn't believe the "safe for democracy" rationale one bit, and asked if the American people really wanted to get into the world's bloodiest war: "Will the President and the supporters of this war bill submit it to a vote of the people before the declaration of war goes into effect?" he said on the Senate floor two days later. "Until we are willing to do that, it becomes us to offer as an excuse for our entry into the war the unsupported claim that this war was forced upon the German people by their government 'without their previous knowledge or approval.'
"Who has registered the knowledge or approval of the American people of the course this Congress is called upon to take in declaring war upon Germany? Submit the question to the people, you who support it. You who support it dare not do it, for you know that by a vote of more than ten to one the American people as a body would register their declaration against it." (La Follette's speech is here.)
President Wilson carried the day. On April 6, 1917, Congress voted for war with Germany, with six senators and 50 congressmen voting no.