Reverse slope defence
A reverse slope defence is a military tactic where a defending force is positioned on the slope of an elevated terrain feature such as a hill, ridge, or mountain, on the side opposite from the attacking force. This tactic hinders both the attacker's ability to observe the defender's positions as well as reducing the effectiveness of the attacker's long-range weapons such as tanks and artillery. Its advantages are negated, however, if the attacker has substantial air presence – even short of air superiority, let alone air supremacy – that enables the attacker to obtain an aerial view over the crest of the terrain feature.
A defending unit usually does not conduct a reverse-slope defence along its entire front; however, there are situations where subordinate units and weapon systems may be positioned on the reverse slope. This may be a wise choice when enemy forces have superior long-range direct-fire or indirect-fire weapons. The defending force uses the hill to limit enemy observation reducing the effectiveness of long-range enemy fire. It may even succeed in deceiving the enemy as to the true location and organisation of the main defensive positions. As the attacker advances and passes over the top of the hill, they may be ambushed by short-range fire from the defender.
The most well-known proponent of the tactic was the Duke of Wellington who used it repeatedly during the Napoleonic Wars to defeat the French infantry tactic of attacking in columns. By placing a ridge between his own army and his opponent's, and having his troops lie down, Wellington was able both to protect his troops from French artillery fire and to surprise the attacking French infantry by having his troops stand up at the last moment and deliver volleys of musketry at close range. So often were the French beaten in this manner it prompted Wellington to comment, "They came on in the same old way and we defeated them in the same old way."
American Civil War
The Battle of Gettysburg, especially the Union defense against Pickett's Charge which was greatly aided by the reverse slope of Emmetsburg Ridge (along the Emmetsburg Road) which both protected and hid both infantry and large numbers of cannons but was not easily discerned by the attackers.
World War II
After the capture of Carentan by American paratroopers, German forces (elements of the 17th SS Panzergrenadier Division and 6th Fallschirmjäger Regiment) counterattacked in an attempt to recapture this strategically vital town on 13 June 1944. Elements of the U.S. 101st Airborne Division (502nd and 506th Parachute Infantry Regiments (PIR)) met the enemy advance southwest of Carentan at the Battle of Bloody Gulch.
The terrain offered the Americans the opportunity of a reverse slope defence and two companies of the 506th PIR lined up along the hedgerows at the bottom of Hill 30. The American troops were outnumbered and being hit with tank and assault gun fire, but the reverse incline enabled them to direct all their firepower at the Germans as they appeared over the top of the hill. Although they were almost overrun, their position gave them enough of an advantage to hold their ground until they were relieved by the U.S. 2nd Armored Division.
Reverse slope defenses were very popular with the Japanese, in the island campaigns of the Pacific war. American superiority in naval support artillery prompted the Japanese to shelter on reverse slopes, until they could engage American troops at close range.
Argentine defensive positions were on the forward slopes. At the Battle of Wireless Ridge, when 2nd Battalion, The Parachute Regiment (2 Para) occupied Argentine positions, they were on a reverse slope from the enemy, protected from Argentine artillery fire. 
At the Battle of 73 Easting , M1A1 tanks of Eagle Troop led by Captain McMaster crested a hill and surprised an Iraqi tank company set up in a reverse slope defence on the 70 Easting. They immediately engaged the Iraqi tanks and destroyed the company.
- Lord Montagu of Beaulieu speaking in the House of Lords Hansard: 24 Apr 1996 : Column 1172, paragraph 4
- Infantry Tactics and Combat during the Napoleonic Wars ~ Part 3 ~ Columns. This article argues that all armies of the period used column formations at times on the battlefield, the military historian Sir Charles Oman is credited with developing the theory that the French practically always attacked in heavy columns, and it is only now that this alleged error, propagated by other British and American authors, is being repudiated. (see also Historical revisionism: French attack formations in the Napoleonic wars)