Question: During which span of years was the war where UN troops approached the Yalu River?

Knowledge 1: Korean War
UN forces retreated to Suwon in the west, Wonju in the center, and the territory north of Samcheok in the east, where the battlefront stabilized and held. The PVA had outrun its logistics capability and thus were unable to press on beyond Seoul as food, ammunition, and matériel were carried nightly, on foot and bicycle, from the border at the Yalu River to the three battle lines. In late January, upon finding that the PVA had abandoned their battle lines, General Ridgway ordered a reconnaissance-in-force, which became Operation Roundup (5 February 1951). A full-scale X Corps advance proceeded, which fully exploited the UN Command's air superiority, concluding with the UN reaching the Han River and recapturing Wonju. [SEP] Knowledge 2: Modern history
The Cold War saw periods of both heightened tension and relative calm. International crises arose, such as the Berlin Blockade (1948–1949), the Korean War (1950–1953), the Berlin Crisis of 1961, the Vietnam War (1959–1975), the Cuban Missile Crisis (1962), the Soviet war in Afghanistan (1979–1989) and NATO exercises in November 1983. There were also periods of reduced tension as both sides sought détente. Direct military attacks on adversaries were deterred by the potential for mutual assured destruction using deliverable nuclear weapons. In the Cold War era, the Generation of Love and the rise of computers changed society in very different, complex ways, including higher social and local mobility.
Knowledge 1: Yalu River
The Korean side of the river was heavily industrialized during the period of Japanese rule (1910 -- 1945), and by 1945 almost 20% of Imperial Japan's total industrial output originated in Korea. During the Korean War, the movement of United Nations troops approaching the river precipitated massive Chinese intervention from around Dandong. In the course of the conflict every bridge across the river except one was destroyed. The one remaining bridge was the Sino -- Korean Friendship Bridge connecting Sinuiju, North Korea to Dandong, China. During the war the valley surrounding the western end of the river also became the focal point of a series of dogfights for air superiority over North Korea, earning the nickname ``MiG Alley ''in reference to the MiG - 15 fighters flown by the combined North Korean, Chinese and Soviet forces. [SEP] Knowledge 2: Modern history
The Cold War saw periods of both heightened tension and relative calm. International crises arose, such as the Berlin Blockade (1948–1949), the Korean War (1950–1953), the Berlin Crisis of 1961, the Vietnam War (1959–1975), the Cuban Missile Crisis (1962), the Soviet war in Afghanistan (1979–1989) and NATO exercises in November 1983. There were also periods of reduced tension as both sides sought détente. Direct military attacks on adversaries were deterred by the potential for mutual assured destruction using deliverable nuclear weapons. In the Cold War era, the Generation of Love and the rise of computers changed society in very different, complex ways, including higher social and local mobility.
Knowledge 1: Yalu River
The Korean side of the river was heavily industrialized during the period of Japanese rule (1910 -- 1945), and by 1945 almost 20% of Imperial Japan's total industrial output originated in Korea. During the Korean War, the movement of United Nations troops approaching the river precipitated massive Chinese intervention from around Dandong. In the course of the conflict every bridge across the river except one was destroyed. The one remaining bridge was the Sino -- Korean Friendship Bridge connecting Sinuiju, North Korea to Dandong, China. During the war the valley surrounding the western end of the river also became the focal point of a series of dogfights for air superiority over North Korea, earning the nickname ``MiG Alley ''in reference to the MiG - 15 fighters flown by the combined North Korean, Chinese and Soviet forces. [SEP] Knowledge 2: Korean War
After the first two months of the conflict, South Korean forces were on the point of defeat, forced back to the Pusan Perimeter. In September 1950, an amphibious UN counter-offensive was launched at Inchon, and cut off many of the North Korean attackers. Those that escaped envelopment and capture were rapidly forced back north all the way to the border with China at the Yalu River, or into the mountainous interior. At this point, in October 1950, Chinese forces crossed the Yalu and entered the war. Chinese intervention triggered a retreat of UN forces which continued until mid-1951. After these dramatic reversals of fortune, which saw Seoul change hands four times, the last two years of conflict became a war of attrition, with the front line close to the 38th parallel. The war in the air, however, was never a stalemate. North Korea was subject to a massive bombing campaign. Jet fighters confronted each other in air-to-air combat for the first time in history, and Soviet pilots covertly flew in defense of their Communist allies.

Answer: 1950–1953)