People have to keep in mind
that our intelligence capabilities in 1950 were not what they are today.
No overflights of the DPRK were possible due to political sensitivities
and the basic fear that a US recon aircraft would get shot down, ergo becoming
the pretext for a "counteroffensive".
Many of the spies did not
survive long enough to get any useful information, and many of the commando
groups were ignored by both ROK and US sources as not trustworthy.
SIGINT was good as far as
it went -- but NSA history notes they only had two linguists in 1950, so
whereas intercept probably wasn't a problem, translation and exploitation
would have been non-existent. Most of our SIGINT aircraft at the time were
ELINT only, which were looking for radar sets. Alfred Price has a good
deal of coverage in his Volume II of the history of US electronic warfare.
Add in the failures at FEC
headquarters and you have the makings of a big intel miss. We weren't fooled,
we really could not "see" it coming.