The world war that lasted from August 1914 to 11 November 1918 was unlike any other conflict in history. No one—neither the leaders nor the civilians of the warring nations—expected it to last so long or to be so brutal. By its end, it would kill nine million people in uniform and an estimated twenty-million more civilians. In the aftermath, our world would be permanently changed.

The conflict was the first global struggle involving industrialized nations. It also marked the first time that countries used airplanes, tanks and submarines in large numbers. And it was the first time that poison gas was used to kill people. Despite the enormous death toll, no one was ready for another world war.

Many hoped that peace would soon prevail, but the situation deteriorated rapidly. Germany and Austria-Hungary were weakened by economic crisis, while Russia was tumbling toward revolution.

The United States, at first hoping to maintain neutrality, became drawn into the war by escalating German aggression, including the sinking of the American ocean liner RMS Lusitania with hundreds of Americans on board. President Woodrow Wilson addressed Congress, requesting a declaration of war.

As the fighting continued, both sides mobilized massive armies of men and horses. By the time it ended, world war had become a much more modern form of warfare, with tanks, airplanes, submarines, machine guns, modern artillery and flamethrowers, as well as poison gas. The war drew soldiers from every corner of the globe.