advance guard

/AH0 D V AE1 N S G AA0 R D/
noun
  1. 1

    A unit or units leading the way of advancing or marching troops, guarding them from ambushes, reconnoitering and pursuing the enemy, etc.

  2. 2

    An individual soldier in the same role.

Source: https://en.wiktionary.org/wiki/advance%20guard