World War I broke out in Europe in the summer of 1914. The war set Germany and Austria-Hungary (the Central Powers) against the United Kingdom, France, and Russia (the Allied Powers), and eventually involved many more nations. The United States declared itself a neutral nation, but neutrality proved elusive. For three years, as Europeans faced war on an unprecedented scale, the neutrality so popular in the United States gradually slipped away.At the outset, Germany and Britain each sought to terminate U.S. trade with the other. Exploiting its naval advantage, Britain gained the upper hand and almost ended U.S. trade with Germany. Americans protested this interference, but when German submarines, known as U-boats, began to use unrestricted submarine warfare in 1915, American public opinion turned against Germany. attacks.