Marching Girls involves competitive teams of young women engaging in precision marching wearing military-style uniforms. It began in the 1930s during the Great Depression as a way to encourage teenage girls to keep fit and healthy. It has its predecessors in various similar male activities - eg, boys' cadets, bugle corps, brass bands etc. - in an earlier age when militaristic training and patriotic fervor were felt to be good for one's character.
The numbers participating in Marching Girls have declined in recent years, reflecting competing claims for young women's time & attention, no doubt. But competitions still draw teams and a crowd, even if somewhat smaller than yesteryear.
Pacific Beat Street takes a look at Marching Girls (audio & pics are annoyingly out of sync, but that doesn't affect what's important - the military precision marching):