The NYT has a picture of a Korean War secret memo that one scholar had found was reclassified. It was an intelligence assessment that said it was "possible but not probable" that China would be capable of intervening on the North Korean side in 1950. It was dated October 12th, 1950. The Chinese sent an Expeditionary Force of 300,000 to Korea 15 days later, on October 27th.
What possible National Security justification could there be to reclassify how badly intelligence analysts failed 55 years ago?
I believe that the sole explanation for reclassifying this information is the same as the real reason for keeping the identities, medical records and allegations against the Guantanamo detainees classified. Embarrassment. Reading the unclassified dossiers that the Associated Press made available for download leaves clear to me that a substantial fraction of the detainees are completely innocent, caught up in a Kafka-esque nightmare.
Here is the key passage from the reclassified memo:
While full-scale Chinese Communist intervention in Korea must be regarded as a continuing possibility, a consideration of all known factors leads to the conclusion that barring a Soviet decision for global war, such action is not probable in 1950. During this period intervention will probably be confined to continued covert assistance to the North Koreans.