Margaret Hall wanted to see the world. When the U.S. entered the Great War, she had her chance. In August 1918, she sailed for France to volunteer for the American Red Cross, an experience documented in her memoir "Letters and Photographs from the Battle Country." Hall embraced the new opportunities for women that the war offered, but the sight of a "girl porter" carrying her heavy baggage in Paris gave her pause. "Felt like a criminal," she wrote; "it was that or nothing, though."

A girl or nothing: The men were gone. Many would return maimed or not at all. So women worked as nurses, chauffeurs and clerks and on assembly lines. In Britain, the estimated 1 million women in munitions factories were known as "munitionettes" or—because the TNT often turned their skin yellow—as "canaries." In France, one commander estimated that if women in arms factories halted work for 20 minutes, the war would be lost.