The politics of the period were dominated by the wishes of the Grandees (Senior Officers) of the New Model Army and their civilian supporters. They encouraged (or at least tolerated) several republican regimes.
From 1649 until 1653 executive powers lay with Council of State, while legislative functions were carried out by the Rump Parliament.
In 1653 the Grandees, with Oliver Cromwell in the lead, dismissed the Rump, and replaced it with a Nominated Assembly (nicknamed the Parliament of Saints and Barebone's Parliament) made up of 140 nominees, 129 from England and Wales, five from Scotland and six from Ireland. It proved to be as difficult for the executive to work with this parliament as it had with the Rump, so, after sitting for five months, members friendly to the Grandees engendered its dissolution on 12 December 1653.
The Instrument of Government was adopted on 15 December 1653 and the pre-eminent Grandee Oliver Cromwell was installed as Lord Protector on the following day. The Instrument of Government granted executive power to the Lord Protector. Although this post was elective, not hereditary, it was to be held for life. It also required the calling of triennial Parliaments, with each sitting for at least five months.
In January 1655, Cromwell dissolved the first Protectorate Parliament, ushering in a period of military rule by the Major Generals.
The Instrument of Government was replaced in May 1657 by England's second, and last, codified constitution, the Humble Petition and Advice. However Oliver Cromwell died the next year and his nominated successor as Lord Protector, his son Richard, proved unable to govern effectively as various political parties strove to gain power.
The Protectorate came to an end in May 1659 when the Grandees recalled the Rump Parliament, which authorised a Committee of Safety to replace Richard's Council of State. This ushered in a period of unstable government, which did not come to an end until February 1660 when General George Monck, the English military governor of Scotland, marched to London at the head of his troops, and oversaw the restoration of the monarchy under Charles II.