When World War I broke out in Europe during the summer of 1914, the United States stuck to a strict policy of neutrality. On August 4, 1914, President Woodrow Wilson declared to Congress that, "The United States must be neutral in fact, as well as in name, during these days that are to try men's souls. We must be impartial in thought, as well as action." However, after years of unrestricted German submarine warfare, the sinking of the Lusitania, the Black Tom explosion, and the public release of the Zimmermann Telegram, the United States could no longer remain neutral in the international conflict. On April 6, 1917, Congress officially declared war against Germany and joined the Allied forces.