advance guard

a body of troops going before the main force to clear the way, guard against surprise, etc.

Origin of advance guard

First recorded in 1750–60

It has had its advance-guard, its rear-guard, and its stragglers.

What Social Classes Owe to Each Other |William Graham Sumner
The bomb had been exploded over the advance-guard, and had destroyed it utterly.

Pony Tracks |Frederic Remington
The advance-guard of the expected procession now appeared in the great gateway, a troop of halberdiers.

The Prince and The Pauper, Complete |Mark Twain (Samuel Clemens)
The advance-guard crossed first, and after them the wounded were carried to the opposite shore.

Red Eagle and the Wars With the Creek Indians of Alabama. |George Cary Eggleston

Up to this time Cyrus had met with no opposition, though he was daily expecting to see the advance-guard of his brother's army.

The Two Great Retreats of History |George Grote

advance guard

a military unit sent ahead of a main body to find gaps in enemy defences, clear away minor opposition, and prevent unexpected contact

a temporary military detachment sent ahead of a force to prepare for a landing or other operation, esp by making reconnaissance