 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.