Korean War

Military troops assisting South Korea, fighting as part of the UN's response to North Korea's invasion, had crossed the 38th Parallel into North Korean territory. American combat forces were among them.

In late October of 1950, unseen by UN personnel, thousands of Communist Chinese Forces (referred to as "CCF") crossed (this is a Chinese-perspective link) the China-Korea border and slipped into North Korea from Manchuria. By early November, those Chinese soldiers had nearly crushed the ROK troops and attacked the surprised Americans:

They came out of the hills near Unsan, [126 longitude, 40 latitude] North Korea, blowing bugles in the dying light of day on 1 November 1950, throwing grenades and firing their "burp" guns at the surprised American soldiers of the 8th Cavalry Regiment, 1st Cavalry Division. Those who survived the initial assaults reported how shaken the spectacle of massed Chinese infantry had left them. Thousands of Chinese had attacked from the north, northwest, and west against scattered U.S. and South Korean (Republic of Korea or ROK) units moving deep into North Korea. The Chinese seemed to come out of nowhere as they swarmed around the flanks and over the defensive positions of the surprised United Nations (UN) troops.

Were South Korean and American troops, nearly overrun by the enemy, able to withstand this surprise attack?

Within hours the ROK 15th Regiment on the 8th Cavalry's right flank collapsed, while the 1st and 2d Battalions of the 8th Cavalry fell back in disarray into the city of Unsan. By morning, with their positions being overrun and their guns falling silent, the men of the 8th Cavalry tried to withdraw, but a Chinese roadblock to their rear forced them to abandon their artillery, and the men took to the hills in small groups. Only a few scattered survivors made it back to tell their story.

The next morning, in the midst of mass confusion, one of many mistakes cost even more lives as the Chinese attacked again:

The remaining battalion of the 8th Cavalry, the 3d, was hit early in the morning of 2 November with the same "human wave" assaults of bugle-blowing Chinese. In the confusion, one company-size Chinese element was mistaken for South Koreans and allowed to pass a critical bridge near the battalion command post (CP). Once over the bridge, the enemy commander blew his bugle, and the Chinese, throwing satchel charges and grenades, overran the CP.

As bad as things seemed, they would get worse.