Combat engineers, also called sappers or pioneers, are land forces troops with basic infantry skills, whose major responsibility on the battlefield involves building and breaching obstacles, land mines and demining, explosives and demolition, river crossing using mobile bridges, and related technical functions.They may also be qualified in conventional civil engineering such as road construction, but the difference between a combat engineer and a general engineer is that the former is trained and equipped to operate under fire, and in close cooperation with other combat arms.
The term "sap" goes back to medieval fortifications and sieges, and refers to digging under fortifications, either to make them collapse or to provide an entry for one's own troops. During the Vietnam War, "sapper" was an elite designation among the Viet Cong, who would infiltrate enemy positions and hand-emplace explosive charges. Rudyard Kipling memorialized the British usage:
When the Waters were dried an' the Earth did appear, ("It's all one," says the Sapper), The Lord He created the Engineer, Her Majesty's Royal Engineer, With the rank and pay of a Sapper! 
French combat engineers are called sapeurs.
The term United States Army Corps of Engineers is somewhat confusing, as it encompasses both large civil construction projects such as flood control in the United States, as well as the Engineer Branch of the U.S. Army. Members of the latter are considered combat arms troops, and, in the Restructuring of the United States Army, are assigned as engineer companies of Heavy Brigade Combat Teams or to Combat Support Brigades. In the latter, they work with other services in a multidisciplinary approach to enhancing mobility. For example, before engineers can repair a bridge, chemical corps personnel might need to decontaminate it. While the engineers are repairing it, air defense artillery units of the brigade prevent enemy air and missile attacks on them.
Individual skills are under Career Management Field 21, with a variety of Military Occupational Specialties.  In 2004, qualified engineers joined the Rangers and United States Army Special Forces in having the right to an elite uniform shoulder tab, Sapper.
While the U.S. Marine Corps have Pioneer units, the U.S. Navy's Naval Construction Battalions "Seabees" have a long tradition of combat engineering. Officially, the motto of the Army Engineers is "Essayons", from the French for "let us try", the unofficial motto of the Engineers and the Seabees is "first we dig 'em, then we die in 'em".