Prospective service members are often recruited from high school or college, the target age ranges being 18–35 in the Army, 18–28 in the Marine Corps, 18–34 in the Navy, 18–39 in the Air Force and 18–27 (up to age 32 if qualified for attending guaranteed "A" school) in the Coast Guard. With the permission of a parent or guardian, applicants can enlist at age 17 and participate in the Delayed Entry Program (DEP), in which the applicant is given the opportunity to participate in locally sponsored military activities, which can range from sports to competitions led by recruiters or other military liaisons (each recruiting station's DEP varies).
The increasing importance of cinema in the early 20th century provided a new platform for depictions of military subjects. During the First World War, although heavily censored, newsreels enabled those at home to see for themselves a heavily sanitised version of life at the front line. About the same time, both pro-war and anti-war films came to the silver screen. One of the first films on military aviation, Hell's Angels, broke all box office records on its release in 1929. Soon, war films of all types were showing throughout the world, notably those of Charlie Chaplin who actively promoted war bonds and voluntary enlistment.
The process of allocating resources is conducted by determining a military budget, which is administered by a military finance organisation within the military. Military procurement is then authorised to purchase or contract provision of goods and services to the military, whether in peacetime at a permanent base, or in a combat zone from local population.
As the speed of technological advances accelerated in civilian applications, so too warfare became more industralised. The newly invented machine gun and repeating rifle redefined firepower on the battlefield, and, in part, explains the high casualty rates of the American Civil War. The next breakthrough was the conversion of artillery parks from the muzzle loading guns, to the quicker loading breech loading guns with recoiling barrel that allowed quicker aimed fire and use of a shield. The widespread introduction of low smoke (smokeless) propellant powders since the 1880s also allowed for a great improvement of artillery ranges.
The last requirement of the military is for military performance assessment, and learning from it. These two functions are performed by military historians and military theorists who seek to identify failures and success of the armed force, and integrate corrections into the military reform, with the aim of producing an improved force capable of performing adequately, should there be a national defence policy review.
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.