The the forward defense strategy in OPLAN 5027 was developed by Combined Forces Commander US General James F. Hollingsworth in 1973 [this discussion is based on "Winning in Korea Without Landmines," by Caleb Rossiter]. Prior to this time, OPLAN 5027 focused primarily on defeating a North Korean invasion. It envisioned the allies staging a 50-mile fighting retreat along the primary armored invasion route from the Demilitarized Zone (DMZ), and taking up strong positions [the "Hollingsworth Line"] south of the broad Han River where it bisects the South Korean capital of Seoul. There, allied forces would wait for US reinforcements before counter-attacking.
Concerned that the US withdrawal from Vietnam might lead the DPRK to question American commitment to defend South Korea, Hollingsworth altered the focus of OPLAN 50-27 to a forward-based offensive strategy. The goal was to convince North Korea that an invasion could bring an end to its regime. The new posture moved most allied artillery, tanks,and infantry forward toward the Military Control Zone (MCZ), which runs five miles south of the DMZ. General Hollingsworth announced plans to strike north after these forces defeated the invasion. He assigned two brigades of the US 2nd Division to seize the North Korean staging city of Kaesong just across the DMZ, and promised around-the-clock raids on the North by B-52 bombers and a "violent,short war " to capture the capital of Pyongyang.
It was unclear whether Hollingsworth's plans included the use of the US tactical nuclear weapons then on the Korean peninsula if the North Korean invasion forces overwhelmed the allies. At the time, the Congressional Budget Office predicted that without nuclear weapons,the new strategy could result in the initial loss of Seoul. In 1975 Gen. Hollingsworth stated that the US had a '9-day war plan', according to which North Korea would be defeated in a few days in a violent clash with 700-800 air sorties.
