When World War I broke out President Wilson vowed to keep the United States out of the war. It tried to implement a policy of neutrality. However with the British controlling the seas the United States willingness to sell arms to anyone who could pay it in fact became the major supplier of arms only to the Allies. The German one effective naval weapon against the superior British navy and that was its submarines. The German use of submarines contravened international maritime law and was strongly opposed by the United States. The sinking of the Lusitania in 1915 started turning US opinion against Germany. The Germans attempted to moderate their submarine campaign, however after their failure to win a victory in the naval battle of Jutland the Germans announced that they would resume unrestricted submarine warfare. That announcement resulted in the United States cutting off diplomatic relations with Germany.
The Germans fearing war with the United States once they began unrestricted warfare made that war inevitable when they sent what has become known as the Zimmerman telegram to the Mexico. In that telegram they encouraged Mexico to go war with the United States. The telegram stated:
We intend to begin on the first of February unrestricted submarine warfare. We shall endeavor in spite of this to keep the United States of America neutral. In the event of this not succeeding, we make Mexico a proposal of alliance on the following basis: make war together, make peace together, generous financial support and an understanding on our part that Mexico is to reconquer the lost territory in Texas, New Mexico, and Arizona. The settlement in detail is left to you. You will inform the President of the above most secretly as soon as the outbreak of war with the United States of America is certain and add the suggestion that he should, on his own initiative, invite Japan to immediate adherence and at the same time mediate between Japan and ourselves. Please call the President's attention to the fact that the ruthless employment of our submarines now offers the prospect of compelling England in a few months to make peace.
Unbeknownst to the German the British intelligence services had broken the German diplomatic code and were able to provide the US the transcript of the communication. Once the Germans resumed their unrestricted submarine warfare public opinion turned strongly against the Germans. There was also fear among foreign policy experts that after their victory against the Russians the Germans were in a position the achieve victory against the British and French and that they felt would not be in the US interest.
Finally on April 2, 1917 President Wilson appeared before a joint session of Congress asking for a declaration of war against Germany. He ended his address by saying :
“There are, it may be, many months of fiery trial and sacrifice ahead of us. It is a fearful thing to lead this great peaceful people into war, into the most terrible and disastrous of all wars, civilization itself seeming to be in the balance. But the right is more precious than peace, and we shall fight for the things which we have always carried nearest our hearts,— for democracy, for the right of those who submit to authority to have a voice in their
own Governments, for the rights and liberties of small nations, for a universal dominion of right by such a concert of free peoples as shall bring peace and safety to all nations and make the world itself at last free. To such a task we can dedicate our lives and our fortunes, everything that we are and everything that we have, with the pride of those who know that the day has come when America is privileged to spend her blood and her might for the principles that gave her birth and happiness and the peace which she has treasured. God helping her, she can do no other.”
On April 6th the Congress declared war. In the Senate the vote was 82-6 in the House 373-50