boots on the ground

Short definition from Wiktionary

Noun

1. 1
   The ground forces actually fighting in a war or conflict, rather than troops not engaged or other military action such as air strikes.
2. 2
   The ground forces actually fighting in a war or conflict, rather than troops not engaged or other military action such as air strikes: The military policy of using ground forces.
3. 3
   Personnel operating in an area of interest.
4. 4
   The state or condition of being physically present at a place of interest in order to ascertain the current situation.
5. 5
   Used other than figuratively or idiomatically: see boots, on, ground.