The Commonwealth of England, Scotland and Ireland was established as a republic following the end of the Second English Civil War and the trial and execution of Charles I. "An Act declaring England to be a Commonwealth", adopted by the Rump Parliament on 19 May 1649.
Oliver Cromwell was declared Lord Protector of a united "Commonwealth of England, Scotland and Ireland" under the terms of the Instrument of Government, inaugurating the period now usually known as the Protectorate. After Cromwell's death, and following a brief period of rule under his son, Richard Cromwell, the Protectorate Parliament was dissolved in 1659 and the Rump Parliament recalled, the start of a process that led to the restoration of the monarchy in 1660.
The Commonwealth lasted from the execution of Charles I in January, 1649 until Charles II entered London in May, 1660.