What chain of events finally moved President Wilson to ask Congress for a declaration of war against Germany?
The chain of events that led to President Wilson declaring war against Germany involved aggression against United States' interests. Wilson previously ran for President on the slogan that he would "keep us out of war." This remained true as Wilson sought to establish peace and diplomatic dialogue with Germany. Even with the sinking of the passenger ship, the Lusitania, Wilson sought to broker a peace with Germany. Wilson demonstrated this in trying to establish an agreement between Central Powers nations and Allied Forces nations in Europe.
However, German aggression continued. Germany made it clear that it wanted to establish naval control of the seas. This extended into targeting United States submarines. Germany also sought to target United States maritime interests. Such targeting impacted Wilson's resolve towards diplomacy and dialogue with Germany. Germany's aggressive stance continued in the British interception of the Zimmerman Telegram. This document suggested a potential German- Mexican alliance against the United States. The revelation from the Zimmerman Telegram further moved Wilson towards a hardened stance against Germany. German aggression continued on throughout the late winter and into spring, when German submarines torpedoed an American military carrier. The deaths of American servicemen moved Wilson into a point where he felt the need to ask Congress to declare war.
The chain of events that moved America to war was German aggression. When Wilson asks Congress to declare war, Wilson sites this as the reason why America had no choice but to declare war:
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.
Wilson makes the case that the cause for war is for "the vindication of right." This "human right" is the right to live free from aggression and harassment from other nations. German aggression becomes the primary cause for Wilson's need to ask Congress to declare war.