Because of the threat of merchant ships being sunk by U-boats, but also because the US didn't want to support the possible enemy (depending on what war you are talking about). The US also became very anti-German, even re-naming the hamburger and other foods.
Germany lost to the allies in WW1 and suffered huge territorial losses giving away its land and population to Poland, Russia, France, Belgium an Denmark and ultimately had to sign The Treaty of Versailles.
