The NYT article on Venona is long, but excellent..
Here is a clip..
It is doubtful that Stalin, rarely a risk-taker, would have supplied the military wherewithal and authorized North Korea to invade South Korea in 1950 had the Soviet Union not exploded an atomic bomb in 1949.
“The Soviet Union’s unrestrained espionage against the United States from 1942 to 1945 was of the type that a nation directs at an enemy state. By the late 1940s the evidence provided by Venona of the massive size and intense hostility of Soviet intelligence operations caused both American counterintelligence professionals and high-level policy-makers to conclude that Stalin had already launched a covert attack on the United States. “