You're FOS.
It's called a prerequisite.
pre·req·ui·site
[prēˈrekwəzət]
NOUN
A thing that is required as a prior condition for something else to happen or exist.
A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed.
"A well regulated Militia, being necessary to the security of a free State", is a prerequisite.
The history of militia in the
United States dates from the colonial era, such as in the American Revolutionary War.
Based on the English system, colonial militias were drawn from the body of adult male citizens of a community, town, or local region. Because there was no standing English Army before the
English Civil War, and subsequently the English Army and later the British Army had few regulars garrisoning North America, colonial militia served a vital role in local conflicts, particularly in the
French and Indian Wars.
Before shooting began in the
American War of Independence, American revolutionaries took control of the militia system, reinvigorating training and excluding men with
Loyalist inclinations.
Regulation of the militia was codified by the
Second Continental Congress with the
Articles of Confederation. The revolutionaries also created a full-time regular army—the
Continental Army—but, because of manpower shortages, the militia provided short-term support to the regulars in the field throughout the war.
With the
Constitutional Convention of 1787 and Article 1 Section 8 of the
United States Constitution, control of the army and the power to direct the militia of the states was concurrently delegated to the federal
Congress.
The
Militia Clauses gave Congress authority for "organizing, arming, and disciplining" the militia, and "governing such Part of them as may be employed in the Service of the United States", and the States retained authority to appoint officers and to impose the training specified by Congress.