During World War I, theatre people pitched in to entertain American soldiers abroad. With the support of the YMCA, actors and variety artists traveled to perform at the front, in hospitals, and at 39 leave areas established in France for the allotted one week off for every four months of active duty in the Army. According to Joe Laurie Jr.'s Vaudeville, the idea for the league originated with E. H. Sothern and Winthrop Ames. Presiding at an organizational meeting on 23 April 1918 at the Palace Theatre, George M. Cohan asked who would be willing to go "over there" during wartime and the entire audience stood up to volunteer.
The Historical Dictionary of the American Theater. James Fisher.