Prior to entering World War I, the United States maintained an official policy of neutrality, though it had closer economic ties to the Allied forces. As the war continued, tensions rose due to attacks on U.S. merchant ships by German U-boats and the publication of the Zimmerman Telegram. Public opinion began to shift in favor of entering the war, and in April 1917 President Wilson asked Congress for a declaration of war against Germany. U.S. involvement helped strengthen the Allies as Russia withdrew from the war, leaving the Western Front as Germany's sole battleground.