American forces, unable to repulse North Korean armor, soon found themselves in an intense urban fight as columns of North Korean troops, spearheaded by T-34 tanks, entered the town from two directions, cutting off U.S. forces.

On the night of June 25, 1950, 10 divisions of the North Korean People's Army launched a full-scale invasion of the nation's neighbor to the south, the Republic of Korea.

The force of 89,000 men moved in six columns, catching the Republic of Korea Army completely by surprise, resulting in a disastrous rout for the South Koreans, who were disorganized, ill-equipped, and unprepared for war.

United States President Harry S. Truman subsequently ordered ground troops into the nation.

[5] However, U.S. forces in the Far East had been steadily decreasing since the end of World War II five years earlier.

At the time, the closest forces were the 24th Infantry Division of the Eighth United States Army, which was performing occupation duty in Kyushu, Japan under the command of William F. Dean.

Most of the 24th Infantry Division's equipment was antiquated due to reductions in military spending following World War II.

[8] During that time, the 34th Infantry Regiment set up a line between the towns of Pyongtaek and Ansong, 10 mi (16 km) south of Osan, to fight the next delaying action against the advancing North Korean forces.

[11] After a 30-minute fight, the battalion mounted a disorganized retreat, with many soldiers abandoning equipment and running away without resisting the North Korean forces.

Also at the town were elements of the 1st Battalion, 21st Infantry that had not made up Task Force Smith at the Battle of Osan.

[13] Shortly before noon, 1st Battalion, 21st Infantry was ordered to withdraw southeast to Chochiwon to keep the railway and supply line to Chonan open.

Large numbers of troops were now located in Myang, Myon, and Songhwan-ni villages, moving to flank Chonan from both sides.

[25][26] The 1st Battalion, still disorganized and under-equipped after its engagement at Pyongtaek the day before, remained in defensive positions south of the town.

[25] The 63rd Field Artillery continued to fire white phosphorus throughout the night, illuminating the terrain for the U.S. forces and preventing them from being overrun.

Five or six tanks at the head of the column infiltrated Chonan and began destroying all vehicles in sight and any buildings suspected of harboring Americans.

[28] After Martin's death, the 3rd Battalion, 34th Infantry, began to buckle as increasing numbers of North Korean troops flooded into Chonan from the northwest and eastern roads.

[28] The 34th Infantry pulled back to the Kum River, its two battalions having been mauled in the battles of Pyongtaek and Chonan.