Myself I'm agnostic, but the sources are MacArthur's biographer who was present when the message to Tokyo Headquarters was relayed to its second in command. All this can be seen in IF Stone's Hidden History... . The other source, of course is the North Korean government, which seems no less reliable than the South in this question.
Whether it was likely that the South would have attacked first, I have to say yes it is more than likely, given the stated intentions to do so, and, more importantly, the desperation of the Rhee regime following its electoral humiliation, rural revolts and the defection of two army brigades northwards.
The strongest evidence against is Kruschev's memoir in which he blames the North Koreans for precipitating the conflict and dragging the USSR in. However, this source has its drawbacks.
I don't set so much store by who fired the first shot. The important question is that the USA was already prepared to use the conflict to justify NSC 68.
-- James Heartfield
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