The obligation to serve in the militia in England derives from a common law tradition, and dates back to Anglo-Saxon times. The tradition was that all able-bodied males were liable to be called out to serve in one of two organisations.These were the posse comitatus, an ad hoc assembly called together by a law officer to apprehend lawbreakers, and the fyrd, a military body intended to preserve internal order or defend the locality against an invader. The latter developed into the militia, and was usually embodied by a royal warrant. Obviously, service in each organisation involved different levels of preparedness.
With the decay of the feudal system and the military revolution of the sixteenth century, the militia began to become an important institution in English life. It was organized on the basis of the shire county, and was one of the responsibilities of the Lord Lieutenant, a royal official (usually a trusted nobleman). Each of the county hundreds was likewise the responsibility of a Deputy Lieutenant, who relayed orders to the justices of the peace or magistrates. Every parish furnished a quota of eligible men, whose names were recorded on muster rolls. Likewise, each household was assessed for the purpose of finding weapons, armour, horses, or their financial equivalent, according to their status. The militia was supposed to be mustered for training purposes from time to time, but this was rarely done. The militia regiments were consequently ill-prepared for an emergency, and could not be relied upon to serve outside their own counties.
Even in the middle of XVI century the majority of the militia was armed with longbows and bills. The 1558 Militia Act sought to increase the presence of pikes and arquebuses in militia armouries. In 1573 trained bands were created, which were armed with pike and shot. These were formed into trained band regiments, particularly in the City of London, where the Artillery Garden was used as a training ground. The trained bands performed an important role in the English Civil War on the side of parliament, in marching to raise the siege of Gloucester (5 September 1643).