Wiktionary
boots on the ground
n. (context idiomatic military English) The ground forces actually fighting in a war or conflict, rather than troops not engaged or other military action such as air strikes.
Wikipedia
Boots on the Ground
Boots on the Ground: A Month with the 82nd Airborne in the Battle for Iraq is a book written by journalist Karl Zinsmeister, who was embedded with the storied 82nd Airborne Division during the early days of Operation Iraqi Freedom.
Usage examples of "boots on the ground".
The crunch of boots on the ground and a grunt from Faile spun him around just in time to be knocked down by the rush of a big shape in a white cloak, a man as thick as Master Luhhan himself.
Drool Rockworm had been able to locate him through the alien touch of his boots on the ground.