The civil war escalated into open warfare when North Korean forces—supported by the SovietUnion and China—moved to the south to unite the country on 25 June 1950. On that day, the United Nations Security Council recognized this North Korean act as invasion and called for an immediate ceasefire. On 27 June, the Security Council adopted S/RES/83: Complaint of aggression upon the Republic of Korea and decided the formation and dispatch of the UN Forces in Korea. Twenty-one countries of the United Nations eventually contributed to the defense of South Korea, with the United States providing 88% of the UN's military personnel.
The first non-Korean and non-US unit to see combat was No. 77 Squadron, Royal AustralianAir Force, which began escort, patrol and ground attack sorties from Iwakuni, Japan
on 2 July 1950.
In five days, the South Korean forces, which had 95,000 men on 25 June, was down to less than 22,000 men. In early July, when U.S. forces arrived, what was left of the South Korean forces were placed under U.S. operational command of the United Nations Command.
The UN ordered an intervention to prevent the conquest of South Korea. U.S. President Harry S. Truman ordered ground forces into South Korea. The 24th Infantry Division was closest to Korea, and it was the first US division to respond. The 24th Division's first mission was to "take the initial shock" of the North Korean assault, then try to slow its advance until more US divisions could arrive.
On 30 June, a 406-man infantry force from 1st Battalion, 21st Infantry Regiment, supported by a 134-man artillery battery (also from the 24th Infantry Division) was sent into South Korea. The force, nicknamed Task Force Smith for its commander, Lieutenant Colonel Charles Smith, was lightly armed and ordered to delay the advance of North Korean forces while the rest of the 24th Infantry Division moved into South Korea. On 4 July, the task force set up in the hills north of Osan and prepared to block advancing North Korean forces. The next day, they spotted an incoming column of troops from the North Korean 105th Armored Division. The ensuing battle was a rout, as the Task Force's obsolescent anti-tank weapons and understrength units were no match for the North Koreans' T-34 Tanks and full-strength formations. Within a few hours, the first battle between American and North Korean forces was lost. Task Force Smith suffered 20 killed and 130 wounded in action. Dozens of US soldiers were captured, and when US forces retook the area, some of the prisoners were discovered to have been executed. According to recently declassified documents the troops were captured and taken to Pyongyang where they are thought to have been murdered about three months later.