The breakup of the Soviet Union in the early 1990s opened previously-closed archives that contained information on Soviet-North Korean-Chinese
relations during the Cold War—of particular interest, correspondence between Josef Stalin, Mao Zedong and Kim Il Sung.

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SOURCE: Neglected Questions on the “Forgotten War”: South Korea and the United States on the Eve of the Korean War

It is well known that South Korean President
Syngman Rhee equaled his North Korean
counterpart’s ambitions to use military force to
reunite his homeland, and that the United
States was determined to prevent his doing so
on his own.

Contrary to traditional views that charged Stalin with
initiating plans for North Korea’s southern
attack, the documents portray Kim Il Sung as
eager to initiate war with South Korea and
Stalin as reluctant to give Kim the green light
to attack. The war’s origins are thus rooted in
Korean nationalist sentiment rather than as
part of a Soviet-led global communist
revolution.

The documents also suggest that
Stalin offered Kim his blessing to attack
sometime in late 1949, but cautioned that the
Soviet Union would not participate beyond
supplying North Korea with weapons. As a
precautionary step he urged the North Korean
leader to approach Mao for any further
assistance he needed.

For an excellent discussion of the issues on the eve of the Korean War, read this article from the Asia-Pacific Journal:

Neglected Questions on the “Forgotten War”: South Korea and the United States on the Eve of the Korean War