The division of Japan's former Korean colony into two zones at the end of World War II led to years of border skirmishing between U.S.-allied South Korea and Soviet-allied North Korea.
On June 25, 1950, the North Korean Army invaded the south to try to reunify the peninsula, beginning the Korean War.

The invasion caught South Korea and its American ally by surprise, and sent the defending South Korean forces into retreat.
 The U.S. moved troops from Japan to fight alongside the South Koreans.
The first troops landed on July 1, and by July 22, three U.S. Army divisions were in Korea, including the 1st Cavalry Division.
 These American troops were insufficiently trained, poorly equipped, and often led by inexperienced officers.
Of particular relevance was that they lacked training in dealing with war-displaced civilians.
The combined U.S. and South Korean forces were initially unable to stop the North Korean advance, and continued to retreat throughout July.

In the two weeks following the first significant U.S. ground troop engagement on July 5, the U.S. Army estimated that 380,000 South Korean civilians fled south, passing through the retreating U.S. and South Korean lines.
 With gaps in their lines, U.S. forces were attacked from the rear, and reports spread that disguised North Korean soldiers were infiltrating refugee columns.
Because of these concerns, orders were issued to fire on Korean civilians in front-line areas, orders discovered decades later in declassified military archives.
Among those issuing the orders was 1st Cavalry Division commander Maj.
Gen. Hobart R. Gay, who deemed Koreans left in the war zone to be "enemy agents", according to U.S. war correspondent O.H.P. King and U.S. diplomat Harold Joyce Noble.
On the night of July 25, that division's 2nd Battalion, 7th Cavalry Regiment, hearing of an enemy breakthrough, fled rearward from its forward positions, to be reorganized the next morning, digging in near the central South Korean village of No Gun Ri.
Later that day, on July 26, 1950, these troops saw hundreds of refugees approaching, many from the nearby villages of Chu Gok Ri and Im Ke Ri.
The division of Japan's former Korean colony into two zones at the end of World War II led to years of border skirmishing between U.S.-allied South Korea and Soviet-allied North Korea. On June 25, 1950, the North Korean Army invaded the south to try to reunify the peninsula, beginning the Korean War.
The invasion caught South Korea and its American ally by surprise, and sent the defending South Korean forces into retreat.  The U.S. moved troops from Japan to fight alongside the South Koreans. The first troops landed on July 1, and by July 22, three U.S. Army divisions were in Korea, including the 1st Cavalry Division.  These American troops were insufficiently trained, poorly equipped, and often led by inexperienced officers. Of particular relevance was that they lacked training in dealing with war-displaced civilians. The combined U.S. and South Korean forces were initially unable to stop the North Korean advance, and continued to retreat throughout July.
In the two weeks following the first significant U.S. ground troop engagement on July 5, the U.S. Army estimated that 380,000 South Korean civilians fled south, passing through the retreating U.S. and South Korean lines.  With gaps in their lines, U.S. forces were attacked from the rear, and reports spread that disguised North Korean soldiers were infiltrating refugee columns. Because of these concerns, orders were issued to fire on Korean civilians in front-line areas, orders discovered decades later in declassified military archives. Among those issuing the orders was 1st Cavalry Division commander Maj. Gen. Hobart R. Gay, who deemed Koreans left in the war zone to be "enemy agents", according to U.S. war correspondent O.H.P. King and U.S. diplomat Harold Joyce Noble. On the night of July 25, that division's 2nd Battalion, 7th Cavalry Regiment, hearing of an enemy breakthrough, fled rearward from its forward positions, to be reorganized the next morning, digging in near the central South Korean village of No Gun Ri. Later that day, on July 26, 1950, these troops saw hundreds of refugees approaching, many from the nearby villages of Chu Gok Ri and Im Ke Ri.
No Gun Ri massacre