With the capitulation of Japan in the Summer of 1945, Korea, which had been under Japanese subjugation for over four decades, was presented with a new regime, as U.S. forces occupied its southern portion and Soviets moved into the north. During the later 1940s, the inital arrangements for a temporary Allied occupation led to a division of the country into two de facto states, neither recognizing the legitimacy of the other and each seeking Korean reunification on its own terms.

In the North, Korean Communists ruled their Democratic People's Republic of Korea (DPRK) with the full support of the Soviets, including substantial military aid. The southern Republic of Korea (ROK), in the hands of more conservative forces, was given modest military support by the United States, but deliberately not enough to produce an offensive capability, or, as events would show, even an effective defensive one.

Until quite late in the pre-war period, South Korea's infant navy received a particularly modest level of support. In fact, the ROK Navy's first significant ship, a former U.S. Navy 173-foot submarine chaser, was purchased on the private market with funds contributed by its own personnel. The United States did provide armament for the vessel, which arrived in Korean waters during the late Spring of 1950. The ROK Navy also had a number of minesweepers, very useful for clearing the explosive left-overs of the great Pacific War, a landing ship and some small craft.

The United States Navy maintained a small fleet in the Western Pacific, with some ships based in the Philippines and others in Japan. By 1950, this force had been reduced to one aircraft carrier and two cruisers, plus a number of destroyers and other ships. Occasionally, these units exercised with their counterparts of Great Britain's Royal Navy. They also maintained a sporadic presence in the troubled waters between mainland China and Taiwan, and "showed the flag" in ports throughout the Western Pacific, including those in South Korea.

In eary 1950, a speech by U.S. Secretary of State Dean Acheson famously omitted the ROK from the United States' defensive interests. A few months later, the DPRK's leader, Kim Il-Sung, shopped around a plan to reunify Korea by force, obtaining the consent of the USSR and China for what then looked like a quick and easy conquest.