The primary reason for the existence of the military is to engage in combat, should it be required to do so by the national defence policy, and to win. This represents an organisational goal of any military, and the primary focus for military thought through military history. How victory is achieved, and what shape it assumes, is studied by most, if not all, military groups on three levels.
Limited Duty Officer: due to the highly technical nature of some officer billets, the Marine Corps, Navy and Coast Guard employ a system of promoting proven senior enlisted members to the ranks of commissioned officers. They fill a need that is similar to, but distinct from that filled by warrant officers (to the point where their accession is through the same school). While warrant officers remain technical experts, LDOs take on the role of a generalist, like that of officers commissioned through more traditional sources. LDOs are limited, not by their authority, but by the types of billets they are allowed to fill. However, in recent times they have come to be used more and more like their more-traditional counterparts.
This continued in the 19th century, with publications like Punch in the British Empire and Le Père Duchesne in France, poking fun at the military establishment. This extended to media other print also. An enduring example is the Major-General's Song from the Gilbert and Sullivan light opera, The Pirates of Penzance, where a senior army officer is satirised for his enormous fund of irrelevant knowledge.
With very few exceptions, becoming a non-commissioned officer (NCO) or petty officer in the U.S. Armed Forces is accomplished by progression through the lower enlisted ranks. However, unlike promotion through the lower enlisted tier, promotion to NCO is generally competitive. NCO ranks begin at E-4 or E-5, depending upon service and are generally attained between three and six years of service. Junior NCOs function as first-line supervisors and squad leaders, training the junior enlisted in their duties and guiding their career advancement.

The Armed Forces Qualification Test (AFQT) is used by all of the Services to determine if an applicant is eligible for the military. Four of the ASVAB subtests are combined to form the AFQT. It measures general cognitive ability and is composed of verbal and mathematics subtests. AFQT scores are grouped into categories for reporting purposes. The table below shows the AFQT categories and the percentile score ranges corresponding to the categories. Applicants that score in AFQT category IIIA or higher may qualify for enlistment incentives.

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