fewer resources and the need to use all air force assets to meet increased operational commitments called for yet another reorganization as the 20th century came to a close. the expeditionary aerospace force concept was implemented to conform to the air force vision to organize, train, equip, deploy and sustain itself in the 21st century global security environment. under the concept, the 4 og is one of two on-call rapid response aerospace expeditionary groups. the fourth was the first to assume this on-call mission on 1 october 1999.