By October 1914 the Allied Powers, Belgium, France, Great Britain, Russia and Serbia were at war with the Central Powers, Austria-Hungary, Germany and the Ottoman Empire. Later, Italy and the United States joined the Allied effort.

To begin the war the Central Powers had the biggest and best-equipped troops. It took the Allied Powers longer to mobilize but they eventually had more manpower and materials.

There were new ways of fighting a war. Warfare looked different; there were trenches, barbed wire, grenades, new artillery, machine guns, tanks, submarines, airplanes and Germany's poison gas. The British Royal Navy established a blockade of Germany and prevented supply ships from reaching Germany.

On May 7, 1915 a German submarine, the U-20, torpedoed the Lusitania, a passenger ship with 128 Americans aboard. The U.S. demanded an end to such attacks. In January 1917, the Germans resumed attacks on U.S. ships again. German submarines fired upon American merchant ships and sank three. In February 1917, a plan to involve Mexico in the war as an ally of Germany’s was uncovered. After the sinking of three American ships, President Woodrow Wilson requested that Congress declare war against Germany. On April 6, 1917 the U.S. joined the war.

In December 1917 the U.S. declared war against Austria-Hungary. Large numbers of U.S. troops were arriving by the summer of 1918. The Germans underestimated the time required to send American troops to Europe.