Key Point: We don't know exactly what would have happened, but it would have deepened American fears about a Communist takeover.
Would the Cold War have been hotter? Would fifty thousand American soldiers have lost their lives? Would today’s Americans worry that North Korean nukes will land on their heads?
The problem with counterfactual history is that it’s often pointless to speculate or the speculation is crazy. But a North Korean victory was more than within the realm of possibility. Kim Il-sung’s army came tantalizingly close to victory in August 1950, when its Soviet-supplied tanks flattened the outgunned and demoralized South Korean troops. Much of the South was captured, before hastily deployed U.S. troops and South Korean remnants barely held onto a bridgehead around the port of Pusan.
Then in September came the U.S. Marine landing at Inchon, and the UN counteroffensive that sent the North Koreans reeling back north across the thirty-eighth parallel. But what if the Marines had never been sent to Korea? The Marines were only there because the Soviet UN ambassador boycotted the Security Council over the issue of Taiwan having a UN seat instead of Communist China. If he had been present, he could have vetoed the first-ever UN resolution authorizing force.
Without a UN mandate, the Truman administration might have balked at sending troops unilaterally. In 1941, U.S. troops had embarked on a crusade against fascism. In 1950, they were fighting a “police action” against aggression (as Alan Alda asked in M*A*S*H: if Korea was a police action, where were the cops?).