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.