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High ground is an area of elevated terrain which can be useful in combat. The military importance of high ground has been recognized for over 2,000 years, for example in Sun Tzu's The Art of War, in which military leaders are advised to take high ground and let the enemy try to attack from a lower position. Fighting from an elevated position is easier for a number of reasons. Holding high ground offers an elevated vantage point with a wide field of view, enabling surveillance of the surrounding landscape, in contrast to valleys which offer a limited field of view.  General Ji Ling of the late Eastern Han Dynasty used this principle to his advantage by sending lookouts to positions of high ground to scout for and provide early warning about enemy troops.  In addition, soldiers fighting uphill will tire more quickly and will move more slowly, while soldiers fighting downhill may not get tired as quickly, and may be able to move faster. Furthermore, soldiers who are elevated above their enemies can get greater range out of low-speed projectiles like rocks, javelins, and grenades. Likewise, low-speed projectiles will have less range when thrown uphill.
Very steep and/or rocky terrain, like mountain sides, can be an obstacle to tanks and armoured personnel carriers, or in the past to cavalry and war elephants. For example, in the Soviet war in Afghanistan, mujahideen guerrillas based themselves in the mountains of Afghanistan, thereby protecting themselves from the Soviet motorized divisions. This forced the Soviets to rely heavily on helicopters to conduct the war, but the United States gave the mujahideen FIM-92 Stinger anti-aircraft missiles, which, arguably, combined with the defense of the mountains, was able to win the war for the mujahideen. The high ground and anti-aircraft missiles made it possible for the mujahideen to use guerrilla warfare against the Soviets without being wiped out. High ground was also employed in the 1423 Battle of Horic in Bohemia, where Taborite soldiers took to high ground, forcing the Utraquist cavalry to dismount to attack them, and also rendering their cannons ineffective. Taborite soldiers were eventually able to make a downhill charge and wipe out the remaining Utraquists. Here again, high ground played a crucial role in the outcome of the battle.
However, getting the high ground is not always advantageous. In Battle of Jieting of the Three Kingdoms period of China, Shu Han forces occupied a hilltop, where Cao Wei forces soon surrounded and isolated the Shu forces from water supplies and reinforcements. The Shu forces suffered a humiliating defeat, and the Shu northern expedition had to be aborted.