In U.S. defense planning parlance, a Major Theater War (MTW) is a contingency that requires the engagement of a significant portion of the United States Armed Forces in a multi-phase assault. War on Iraq 2003 was the first MTW since the term came into use: the original Gulf War also fit the MTW paradigm.

Bill Clinton's administration, which created the MTW concept, directed the Department of Defense to be prepared to fight two simultaneous MTW's: one around the Persian Gulf and one in Korea. George W. Bush's administration changed the requirement to "4-2-1," meaning that the military must be able to hold their ground in four simultaneous MTW's (Europe, the Middle East, South Asia, and East Asia), win at least two of them, and force a regime change in at least one of them.

The basic phases of a major theater war are:

  1. Stopping the enemy advance.
  2. Building up forces.
  3. Attacking.
  4. Stabilizing the area.
There are written battle plans, called OPLANs, for each anticipated MTW. The plan for an Iraqi conflict is OPLAN 1003: the plan for a Korean conflict is OPLAN 5030, which recently replaced OPLAN 5027. Each OPLAN designates specific units to employ in the conflict, and these designated units have to maintain a mandated level of combat readiness at all times.