A combat engineer (also called field engineer, pioneer or sapper in many armies) is a performs a variety of construction and demolition tasks under combat conditions.
German Pioneer Auction.
The combat engineer's goals involve facilitating movement and support
of friendly forces while impeding those of the enemy. Combat engineers
build, repair and maintain buildings, roads and power supplies.
employ explosives for construction and demolition projects, and clear
minefields using sometimes specialized vehicles. Such tasks typically include constructing and breaching trenches, tank traps and other fortifications, bunker construction, bridge and road construction or destruction, laying or clearing land mines and other physical work in the battlefield.
Usually, a combat engineer is also trained as an infantryman and combat engineering units often have a secondary role fighting as
infantry . . . or as we used to call them: an infantryman with a shovel!
Seriously, a good combat engineer is worth his weight in gold.