Trench warfare during World War I

Germany also was at war on its eastern border, with Russia. Losses on that battlefront, too, totaled more than a million men.

MAURICE JOYCE: At the time of America's presidential election in nineteen sixteen, Germany seemed to be winning the war. Its losses were terrible. But the losses of its enemies -- The Allies -- were even worse. German forces occupied much of northern France and almost all of Belgium. German and Austrian soldiers also held parts of Russia, Italy, Romania, and Serbia.

Germany was winning on the battlefield. The Allies were winning at sea. A British blockade cut off almost all German trade with the rest of the world. Even food shipments were blocked. As a result, Germany faced mass starvation. It urgently needed to break the blockade and get food.

LARRY WEST: This situation finally forced Germany to make the decision that would bring the United States into the war.

It decided to use its submarines to break the British blockade. The submarines would attack any ships that came near Britain or other parts of Europe. This included ships from neutral countries, like the United States.

Earlier, Germany had made a promise to the United States. Its submarines would not attack civilian ships unless warning was given and the lives of those on the ships were saved. Now Germany was withdrawing that promise. It said unrestricted submarine warfare would begin immediately.