An ambush is a long-established military tactic in which combatants take advantage of concealment and the element of surprise to attack unsuspecting enemy combatants from concealed positions, such as among dense underbrush or behind hilltops. Ambushes have been used consistently throughout history, from ancient to modern warfare. In the 20th century, an ambush might involve thousands of soldiers on a large scale, such as over a choke point such as a mountain pass, or a small irregular band or insurgent group attacking a regular armed force patrol. Theoretically, a single well-armed and concealed soldier could ambush other troops in a surprise attack. Sometimes an ambush can involve the exclusive or combined use of improvised explosive devices, that allow the attackers to hit enemy convoys or patrols while minimizing the risk of being exposed to return fire.