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 guerrilla band or insurgent group attacking a regular armed force patrol. Ambushes have been used many times in military history.