This “solid intelligence” might be a bit overstated…

By early 1950 military build-up of Democratic People’s Republic of Korea (DPRK) forces and equipment along the 38th parallel was clearly identified by both U.S. Central Intelligence Agency and Army G2 surveillance networks.

However, regarding a possible North Korean attack, the dominant themes in U.S. intelligence analysis were:

  • DPRK forces could not mount a successful attack without Soviet assistance
  • an attack would signal a world-wide Communist offensive
  • emerging from WWII and Civil War, neither the USSR nor Communist China would initiate a worldwide conflict

https://vividmaps.com/korean-war-1950-1953/



Throughout early June 1950, intelligence reports from South Korea and the CIA reported:

  • removal of civilians from the border area
  • restriction of all transport capabilities for military use only
  • movements of infantry and armor units to the border area. 

On June 25, 1950 North Korea invaded South Korea.





In September 1950, two months into the war, a CIA Intelligence Memorandum assumed that the Chinese were already providing covert assistance to the DPRK, but overt assistance by Communist China would require Soviet approval to risk a general war. The CIA memorandum concluded that although reports of Chinese troop buildups along the Manchurian border made intervention possible, there were no direct indications that China would intervene,


Wikipedia


At the end of September 1950, the US Ambassador in Moscow reported that Soviet and Chinese contacts told both the British and Dutch Ambassadors that if foreign troops cross the 38th parallel, China would intervene.

These warnings were ignored, and although General MacArthur was ordered to advance only South Korean troops to the Yalu River,, US-UN forces continued to push the DRPK forces northward.

Source: Two Strategic Intelligence Mistakes in Korea, 1950 — Central Intelligence Agency



Wikipedia

On November 25 1950 a 300,000-man Chinese offensive caught U.N. forces off guard.