In the early morning hours of June 25th, 1950, the Inmun Gun, the North Korean People's Army [NKPA], launched a massive offensive across the DMZ, and began a drive to overrun and conquer the Republic of Korea [ ROK ], and forcibly absorb it. Led by the 105th Tank Brigade [T 34/85 tanks], and supported by large numbers of artillery tubes, as well as YAK fighters, the NKPA infantry, many of them veterans of the Chinese Civil War, sliced through the ROK troops like butter [ made easier by the fact we hadn't supplied the ROKs with planes, tanks, or anti-tank weapons ]. They took Seoul in three days. They drove south, manhandled the first U.S troops they faced, Task Force Smith [whose light bazookas were useless against the Russian built T 34s ], and headed fior the last major port in South Korea, Pusan.
The NKPA was worn down by air strikes, and stopped at the Pusan perimeter by the Eighth Army. As U.N forces arrived in greater numbers, the Eighth broke out of Pusan, while the overall CG, Douglas MacArthur, directed U.S. Marines in the amphibious assault at Inchon, and the drive on Seoul. The NKPA disintegrated and fled north. It might have ended where it began, at the 38th parallel, in September, 1950, except the U.N authorized MacArthur to drive north, despite threats of action by the Chinese if the U.N troops did so.
MacArthur's brilliance was now replaced by MacArthur's hubris. He sent X Corps, under the underperforming Ned Almond to lead an amphibious assault on the east coast of Korea at Wonsan. By the time Almond got there, the port hand been taken by ROKs marching overland. Almond now moved north to the area of the Chosin reservoir, separated by many miles of rough terrain from the rest of Eighth army, which continued north after capturing Pyongyang. Unbeknownst to Almond, he was separated from the Eighth by masses of Red Chinese troops.
MacArthur's J-2, a dunce named Willoughby, who owed his position for sycophancy above and beyond, dismissed intel from the front resulting from captured Chinese that sizable numbers of Chinese were in Korea. Why? Because he knew that wasn't what MacArthur wanted to hear. Result, a Chinese offensive that drove the Allied troops south of Seoul.
And from there on, the war seesawed back and forth around the 38th parallel. MacArthur was relieved for well known reasons. Ridgeway took over. But what was lost in those weeks and months was that the U.S was no longer fighting to win. It was fighting not to lose. And that was a fairly radical departure from all previous U.S wars. Part of it was dealing with allies [ Clement Atlee comes to mind ]. Part of it was war weariness. And part of it was unpreparedness. Truman and his Sec/Def, Louis Johnson, had gutted the military after WW II, and like the current administration, were fixing the budget on the backs of the military. The equipment was old, and it took a while for the new stuff to get in- country. Then there was the massive Soviet presence in Europe, and not too much in its way. So Truman chose to settle for what he could get on the cheap. Unfortunately, cheap costs lives.
Korea ended about where it started, give or take. And the "Forgotten War" was quickly forgotten - except by the guys who fought there, and their families. But it set the precedent, barely noticed at the time. We seldom fight to win anymore. We seldom accomplish what we set out to do because we don't.