In 1949, Congress dragged its feet in considering a $150 million dollar aid bill to South Korea. Syngman Rhee had so often talked about invading North Korea that US leaders feared giving him too much in the way of weapons. For this reason, South Korea was sent only rifles, bazookas, and light artillery; tanks an airplanes were held back. Also by 1949, most of the US military had moved out. Only 500 advisors, known as KMAG (the Korean Military Advisory Group) remained in South Korea, under the command of Brigadier-General William L. Roberts. In January of 1950, the House defeated the Korean Aid Bill by a single vote; Korea was scheduled no to get American Aid for the following year, 1950.

On June 25, 1950 the North Korean army attacked South Korea, crossing the 38th Parallel. Pentagon officials were stunned, and had no immediate contingency plan ready. Some said little could be done, while others suggested it was the beginning of Stalin's plot to take over the world. Truman and his circle of advisers sat firmly in this latter group. Immediately upon the invasion, these advisors discussed the prospect of sending General Douglas MacArthur, the US commander in the Far East, to lead a military response.

The North Korean invaders hoped to take Seoul, the South Korean capital, as quickly as possible. The majority of ROK forces were routed by North Korean troops. Only one ROK division, the 6th, held its ground. John Muccio, the American ambassador to South Korea, quickly reported back to Washington that a "probable" full-scale attack was under way. Meanwhile, Syngman Rhee reacted to news of the invasion by ordering the imprisonment of more South Koreans.

The UN was particularly upset about the North Korean invasion, because it had overseen the elections held in 1948, and did not want to see a war undo that election. UN Secretary-General Trygve Lie called the invasion a "war against the United Nations." Truman hoped to use the UN as an instrument of US power, and UNCOK (the UN Council on Korea) condemned the attack as a "breach of peace". On Nov. 30, 1950, the UN passed a resolution condemning North Korea's actions.

Syngman Rhee's constant threat to invade North Korea actually led to the weakening of South Korea's military weakened, since US leaders were afraid of what he might do given powerful weaponry.

North Korea made the claim that its attack was made in response to an attack by South Korea. Most historians agree that this claim was probably a lie, especially considering how well planned and coordinated North Korea's so-called response was and how ill-prepared South Korea was to face the invasion. In fact, when the invasion occurred, many South Korean officers were on leave, having left their units for the weekend. It seems clear that South Korea was unprepared for North Korea's assault, and that North Korea's claimed "response" was actually a premeditated offensive.

Along with the general unpreparedness of the South Korean army, the South Koreans stood at a further disadvantage in the military encounter. Whereas the South Koreans had little heavy military equipment, the North Koreans were armed with the Soviet T-34 Tank, which, although lined with only medium armor, was nearly impregnable to the ill-equipped ROK army. The North Korean tanks also had a substantial psychological effect on the South Korean forces, many of whom had never seen a tank before.

Although debate exists on whether Stalin helped plan Kim Il-sung's invasion of South Korea, the presence of Soviet tanks certainly implies a degree of Soviet knowledge of the North Korean invasion. It is likely, most historians agree, that Stalin knew of and gave his approval to the attack. Those historians who disagree, however, ask why would Stalin have given a go-ahead for North Korea's invasion? The North had the minerals, hydro-electricity, and warm-water ports Russia wanted. The South offered very little to the USSR. There are several possible explanations for why Stalin might approve of the attack, however. First, Stalin might have wanted all of Korea, since the US had "all" of Japan in its sphere. Second, Stalin might have wanted to take Korea to prevent the US from having such a close base to attack Soviet territory. Third, he might simply have hoped to frighten and embarrass the US. And fourth, with Dean Acheson's comments at the press club claiming a lack of US interest in Korea, Stalin might have believed that faced with such an attack the US would simply give up.

Why did American leaders jump to such extreme conclusions after North Korea's invasion? Particularly alarming were the parallels between Korea and Berlin, both having been arbitrarily split between Communist and anti-communist. Many Americans considered that Korea might actually be a Communist practice-run for an invasion of West Germany. Furthermore, many people in Washington, thinking along the lines of anti- appeasement and NSC-68, figured that the Soviets were using Korea to test America's reaction. They wanted to give Stalin no belief that Communist expansion would go unpunished.

General MacArthur was perhaps the key figure in the first half of the war. An extremely successful general in World War II, he had an enormous ego made even worse by having dictatorial control over Japan during the American postwar occupation. MacArthur was also obsessed with the idea that Asia was more important to US interests than Europe. Because of this, he always bore a grudge against Truman for giving the war in Europe priority over the war in the Pacific. Finally, MacArthur was supported by Republicans and hoped to one day run for President. Thus, MacArthur saw Truman (a Democrat) as a sort of political enemy despite being his commander-in-chief. Some historians have suggested that Syngman Rhee and MacArthur actually conspired to start the Korean War, but this is based on little fact. There is no denying that independent of his politics, MacArthur was a militarily brilliant tactician; his personal beliefs, however, often affected his military decisions. In sum, MacArthur wielded an inordinate amount of influence on the Korean War, and eventually worked himself into a showdown with Truman.

The UN categorized the North Korean attack a "breach of peace" rather than "aggression" (the worst crime a nation can commit according to the UN ideals) because the Soviets argued that the Korean War was actually a civil war. As the internal war of the nation of Korea, the UN would have no right to interfere in this. Americans mostly ignored this interpretation of the Korean War, but there is something to be said for this view. After all, Korea had throughout its history (unlike Vietnam) been a united country, and many Koreans on both sides of the 38th Parallel wanted the country to stand united. Only an arbitrary line drawn by outside powers made North Korea and South Korea into separate nations.