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The first world war began in 1914 during Wilson's first term and Wilson spent the remainder of the term trying to keep America out of the war in Europe. He offered to be a mediator in the dispute, but neither side took his requests seriously. Republicans, led by former President Theodore Roosevelt, strongly criticized Wilson's refusal to build up the U.S. Army in anticipation of America joining in the war. Wilson won the support of the American pacifists by arguing that an army buildup would provoke war.
On December 18, 1916, Wilson's efforts at peace making were rejected by the warring parties. He had asked both sides to state their minimum terms necessary for future security. The Central Powers were not interested because they felt that victory was certain. The Allies required the dismemberment of their enemies' empires. No desire for peace or common ground existed.
German submarines were killing sailors and civilian passengers on passenger liners and non-military ships. Wilson demanded that Germany stop, but he kept the U.S. out of the war even when Germany ignored his demands. Britain had declared a blockade of Germany to prevent neutral ships (including American vessels) from carrying contraband goods to Germany. Wilson mildly protested this and the British recognized the protests as such.
Wilson ran for re-election in 1916 used as his major campaign slogan "He kept us out of war", referring to his administration's avoiding open conflict with Germany or Mexico. But Wilson was careful in his rhetoric. He never promised to keep out of war under all circumstances. In his acceptance speech on September 2, 1916, Wilson warned Germany that submarine warfare that took American lives would not be tolerated. He said "the nation that violates these essential rights must expect to be checked and called to account by direct challenge and resistance. It at once makes the quarrel in part our own." Wilson narrowly won the election, defeating Republican candidate Charles Evans Hughes.
The U.S. had made a declaration of neutrality in 1914 and Wilson had cautioned against taking sides in the war for fear of endangering U.S. interests. The U.S. maintained neutrality despite increasing pressure placed on Wilson after the sinking of the British passenger liner HMS Lusitania with American citizens on board on May 7, 1915. Secretary of State William Jennings Bryan, a non-interventionist, resigned his cabinet post in June 1915. He said, "why be so shocked by the drowning of a few people, if there is to be no objection to starving a nation.” Despite this, Bryan campaigned for Wilson's reelection in 1916.
Wilson found it increasingly difficult to maintain U.S. neutrality after Germany initiated a program of unrestricted submarine warfare early in 1917 that threatened U.S. commercial shipping, in violation of its previous promises not to engage in such action. The last straw was when Germany attempted to enlist Mexico as an ally against the U.S. In January of 1917 the British intercepted a secret diplomatic communication issued from the German Foreign Office in January 1917 that proposed a military alliance between Germany and Mexico in the event of the United States entering World War I against Germany. In the message, Germany proposed that Mexico would recover Texas, Arizona and New Mexico if they agreed. When the contents of this message (known as the "Zimmerman Telegram") became known, they enraged American public opinion, especially after the German Foreign Secretary Arthur Zimmermann publicly admitted the telegram was genuine on March 3. This turned public opinion and helped generate support for the United States declaration of war on Germany.
Wilson saw all of this as justifying the entry of the United States into World War I. He told Congress that he was asking for a declaration of war to make "the world safe for democracy." In a speech to Congress given on April 2, 1917, he said that unless the U.S. threw its weight into the war, western civilization itself could be destroyed. His proposed that this would be the "war to end all war".
On April 6, Congress declared war. In the Senate, the resolution passed 82 to 6, with Senators Harry Lane, William J. Stone, James Vardaman, Asle Gronna, Robert M. La Follette, Sr., and George W. Norris voting against it. In the House, the declaration passed 373 to 50.