All of the branches work together during operations and joint missions in Unified Combatant Commands, under the authority of the Secretary of Defense with the typical exception of the Coast Guard. Each of the Unified Combatant Commands is headed by a Combatant Commander, a senior commissioned officer who exercises supreme command authority per 10 U.S.C. § 164 over all of the forces, regardless of branch, within his geographical or functional command. By statute, the chain of command flows from the President to the Secretary of Defense to each of the Combatant Commanders. In practice, the Chairman of the Joint Chiefs of Staff often acts as an intermediary between the Secretary of Defense and the Combatant Commanders.
 See two documents: Sims, W.H. and Truss, A.R., A Reexamination of the Normalization of Armed Services Vocational Aptitude Battery (ASVAB) Forms 6, 7, 6E, and 7E (Alexandria, VA: Center for Naval Analyses, September 1980); and Laurence, J.H. and Ramsberger, P.F., Low-Aptitude Men in the Military: Who Profits, Who Pays? (New York: Praeger, 1991).
In the 2000s, women can serve on U.S. combat ships, including in command roles. They are permitted to serve on submarines. Women can fly military aircraft and make up 2% of all pilots in the U.S. Military. In 2003, Major Kim Campbell was awarded the Distinguished Flying Cross for landing her combat damaged A-10 Thunderbolt II with no hydraulic control and only one functional engine after being struck by hostile fire over Baghdad.