James Fortune ~ Operation Focus Retina
This was part of an operation, some 20 years after the First Korean War of 1950 - 1953. The North Korean's denounced the exercise as "the most wanton violation of the Korean Armistice agreement and a reckless playing with fire, threatening peace in Asia and the rest of the world." The United States said they came in because of North Korean incursions into the South.
By bringing in this many troops in so short of time, the United States Military was remembering that it took two weeks before reinforcements could be brought to curtail an incursion by the North Korean into South Korea in June 1950.
Often called the 2nd Korean War (1966 - 1969), the new Nixon Administration, may also have began Focus Retina in response to the USS Pueblo incident of Tue 23 Jan 1968. The USS Pueblo was a Navy intelligence ship and was on surveillance off the coast of North Korea 16 miles, clearly in international waters. But North Koreans approached the ship with guns and demanded surrender.
Some critics have said that the new administration was done to reassure the South Koreans that despite the problems in Vietnam, that the United States stood behind it's commitment to South Korea. It also was a test of how quickly troops could be deployed to an area thousands miles away from it's homeland.