Lord Protector was a title that has been used in British constitutional law for the head of state. It was also a particular title for the British heads of state in respect to the established church. It was sometimes used to refer to holders of other temporary posts; for example, a regent acting for the absent monarch.

The Commonwealth was the political structure during the period from 1649 to 1660 when England and Wales, later along with Ireland and Scotland, were governed as a republic after the end of the Second English Civil War and the trial and execution of Charles I. The republic's existence was declared through "An Act declaring England to be a Commonwealth", adopted by the Rump Parliament on 19 May 1649. Power in the early Commonwealth was vested primarily in the Parliament and a Council of State. During the period, fighting continued, particularly in Ireland and Scotland, between the parliamentary forces and those opposed to them, in the Cromwellian conquest of Ireland and the Anglo-Scottish war of 1650–1652.