An ambush is a long-established military tactic in which a combatant uses an advantage of concealment or the element of surprise to attack unsuspecting enemy combatants from concealed positions, such as among dense underbrush or behind mountaintops. 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 irregulars band or insurgent group attacking a regular armed force patrols. 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. History This use by early people of ambushing may date as far back as two million years when anthropologists have recently suggested that ambush techniques were used to hunt large game. One example from ancient times is the Battle of the Trebia river. Hannibal encamped within striking distance of the Romans with the Trebia River between them, and placed a strong force of cavalry and infantry in concealment, near the battle zone. He had noticed, says Polybius, a "place between the two camps, flat indeed and treeless, but well adapted for an ambuscade, as it was traversed by a water-course with steep banks, densely overgrown with brambles and other thorny plants, and here he proposed to lay a stratagem to surprise the enemy". When the Roman infantry became entangled in combat with his army, the hidden ambush force attacked the Roman infantry in the rear. The result was slaughter and defeat for the Romans. Nevertheless, the battle also displays the effects of good tactical discipline on the part of the ambushed force. Although most of the legions were lost, about 10,000 Romans cut their way through to safety, maintaining unit cohesion. This ability to maintain discipline and break out or maneuver away from a kill zone is a hallmark of good troops and training in any ambush situation. (See Ranger reference below). Ambushes were widely utilized by the Lusitanians, in particular by their chieftain Viriathus. Their usual tactic, called concursare, involved repeatedly charging and retreating, forcing the enemy to eventually give them chase, in order to set up ambushes in difficult terrain