The United States’ insufficient investment in security assistance and cooperation to build the capacity of the Republic of Korea’s (ROK) military before the start of the Korean War contributed to the ROK’s inability to deter or effectively conduct military operations. The U.S. investment in security cooperation and strong U.S. policy support in Korea following the Korean War enabled the U.S. to gradually withdraw forces from the peninsula while simultaneously building ROK capabilities and deterring a North Korean invasion. An analysis of the correlation of U.S. and ROK capabilities will potentially help future researchers and analysts to identify the critical point where the minimum amount of U.S. security cooperation efforts will continue to maintain successful deterrence against an invasion from North Korea.