Key Resolve is the name (as of 2015) for the military exercise previously known as RSOI, which stands for Reception, Staging, Onward Movement, Integration (and even earlier as Team Spirit). It is an annual command post exercise (CPX) held by United States Forces Korea, and conducted with the Republic of Korea Armed Forces.
It is conducted between February and April and focuses on United States Pacific Command OPLANs that support the defense of South Korea. Additionally, US units are moved to Korea from other areas and they conduct maneuvers and gunnery exercises. ROK units also conduct maneuvers with some acting as the OPFOR.
Doctrinally, RSOI is detailed in FM 100-17-3, the field manual for RSOI.
This exercise, like the Ulchi-Freedom Guardian (UFG) exercise, regularly leads to accusations by North Korea that it is a prelude to an invasion by the United States and South Korea.