Political map of Europe & the Mediterranean 6 April 1917 (United States Enters the War): With Russia crumbling (February Revolution), Germany moved to increase its pressure on the West. On February 1, it announced unrestricted submarine warfare (Unrestricted submarine warfare) in an attempt to starve Britain into surrender. This aggravated relations with the United States and, when German plans to incite a war between Mexico and the U.S. (Zimmermann Telegram) were uncovered, America declared war.