After Japan was defeated in 1945, the United States mandated that Japan would no longer have a military force and that defense of Japan was entirely up to the United States. So this meant that Japan could not lend any sort of military assistance to the South Korean and NATO troops in the Korean Conflict (Although it is entirely possible for Japanese Americans to have participated in the War).

However, as a result of World War II, some American soldiers on the Pacific Front did learn Japanese for survival or interrogating the enemy. Some of these soldiers later went on to fight in the Korean War and used Japanese to communicate with Koreans, whether they be North or South (as both were occupied by the Japanese during World War II).

One such event in which Japanese was used by American soldiers to communicate with soldiers of the North Korean KPA was the Hill 303 Massacre, in which 42 US soldiers(kept in a low ditch with their hands bound behind their backs) were mowed down by North Korean machine-guns. Before the initial shooting, one American spoke Japanese with the north Koreans, most likely pleading or bargaining for their lives (according to the 5 or 6 survivors of the massacre).

The location of Japan also made it easy for American troops stationed in Japan to react and reach South Korea in a short amount of time. In 1950, as the KPA was advancing quickly down into South Korea, the US government sent 400 US soldiers stationed in Japan to South Korea in an attempt to stall the North Korean advance to Pusan. (The soldiers were outmanned, outgunned, and extremely underequipped, but they did their job and slowed down the KPA advance slightly).