WORLD WAR I
Germany and Austria-Hungary (the Central Powers) were locked in a war with Britain, France, and Russia (the Entente). The actual fighting started in August 1914 after Franz Ferdinand, the prince who was set to become emperor of Austria-Hungary, was assassinated on a visit to Serbia. However, the endless fight to be the leading nation of Europe had been brewing for hundreds of years. This was exactly the kind of war the United States wanted to avoid.
The natural tendency of English-speaking America to side with England was helped along by careful propaganda coming over the only news wire from Europe, which conveniently ran through Britain. Plus, Germans looked like bad guys with their spiked helmets, upturned mustaches, and habit of tromping through neutral countries.
The millions of Americans of German heritage did little cheering for their old homeland; one of the reasons they left Germany in the first place was to avoid all that military bluster. Americans even changed the German names of foods: sauerkraut became victory cabbage, and more Americans started saying "hot dog" instead of "wiener."
Germany paid a penalty for not having a navy as strong as the British. America proclaimed her neutral rights to the seas, in hope of continuing trade with the warring parties who were very much in need of supplies. Meanwhile, Germany and Britain both blockaded each other. Britain used surface ships that could gently force American cargo ships away from Germany and into British ports. Germany used submarines, which could only wave at passing ships or sink them; they weren't big enough to shepherd the American ships to distant German ports or even take on extra passengers if the ships sank. Because this war was the first with submarines, the whole undersea attack policy seemed unfair to many Americans.