A 1915 poster showing a French soldier holding a grenade

HARRY MONROE: At first, the plan worked. Two million German soldiers swept across Belgium and into France. They rushed forward toward Paris, hoping for a fast victory. But the German commanders made a mistake. They pushed their men too fast. When British and French forces struck back -- outside Paris -- the tired and worn German soldiers could not hold their positions.

The battle was fierce and unbelievably bloody. In the end, the Germans were forced to withdraw.

The German withdrawal gave the allies time to prepare strong defenses. There was no chance now for a quick German victory. Instead, it would be a long war, with Germany and Austria facing enemies on two sides. Britain and France were on the West. Russia was on the East.

KAY GALLANT: The Allies took immediate steps to reduce Germany's trade with the rest of the world. The British navy began seizing war supplies found on neutral ships sailing toward German ports. It then expanded its efforts to block food exports to Germany.

The blockade by Britain and the other allies was very successful. Germany faced possible starvation. Its navy was not strong enough to break the blockade with surface ships. Its only hope was to break the blockade with another naval weapon: submarines.

Germany announced that it would use its submarines to sink any ship that came near the coast of Britain. The threat included ships from neutral nations that tried to continue trading with the Allies.