Signs point to North Korea unilaterally launching the invasion. It was not helpful for the USSR and was at a very bad time for the PRC since the war immediately shut down plans to invade Taiwan.
The U.S., especially after Chinese troops entered the war, viewed it as a united and aggressive communist bloc brashly taking over one more country and likely to try more if not resisted. US defense spending shot back up to wartime levels (though far from the WWII peak) and stayed there.
China also viewed it as a feeler for aggression that would go further if not resisted. Both countries were overinterpreting local issues as global ones.
The dramatic reverses were all in the first year, followed by two years of stalemate before the armistice.
