A reasonable estimate of the defense budget cost of protecting the Persian Gulf (and especially of protecting production and transit of oil) would make an important contribution to a rational assessment of the desirability of continuing to include this mission as part of U.S. military strategy. Unfortunately, such an estimate is not easy to produce. Ideally, it would incorporate three sources of defense budget cost: the avoidable future investment to acquire the force structure to perform the mission, the cost of the force posture linked to the mission (the incremental cost of forward basing, specific training exercises that could be cancelled, etc.), and the intermittent cost of military operations in the region (whether surges of forces to deter potential adversaries or actual wars)...