Oliver Cromwell entered Parliament for Huntingdon in 1628 and returned as a member for Cambridge in 1640. A committed Puritan, he opposed Charles I's religious policies and became a military leader after civil war began in 1642. He raised cavalry for Parliament, emphasizing discipline and religious commitment. His troops contributed to victory at Marston Moor in 1644, and he supported the creation of the New Model Army under Sir Thomas Fairfax.

As lieutenant-general, Cromwell helped secure the decisive victory at Naseby in 1645. Continued conflict over religion, army pay, and the constitutional settlement drew him deeper into politics. He opposed royalist uprisings and defeated a Scottish army at Preston in 1648. After the army excluded parliamentary opponents of a trial, he became a leading supporter of prosecuting the king and signed Charles's death warrant in January 1649.

The new Commonwealth sent Cromwell to Ireland, where he captured Drogheda and Wexford in 1649. The killing of defenders and civilians made these campaigns enduring symbols of brutality, followed by a wider conquest and confiscation of Irish Catholic land. In Scotland, he defeated forces supporting Charles II at Dunbar in 1650. Victory at Worcester in 1651 ended the principal military threat to the republic.

Cromwell's search for a stable, godly government repeatedly brought him into conflict with representative institutions. In April 1653 he forcibly dissolved the Rump Parliament. A nominated assembly followed but surrendered its authority later that year. Under the written Instrument of Government, he became lord protector of England, Scotland, and Ireland in December. His executive authority depended formally on a council and Parliament, yet his government remained underpinned by the army.

He dissolved a resistant Parliament in 1655 and introduced regional government through major-generals. In 1657 he declined an offer of kingship but accepted a revised constitutional settlement. His regime permitted considerable diversity among Protestants while excluding Catholics and imposing limits on other dissenters. Abroad, war against Spain brought the capture of Jamaica, and cooperation with France produced gains at Dunkirk. Cromwell died in 1658 and was succeeded by his son Richard, whose government soon collapsed. His career left a contested legacy of military innovation, religious aspiration, constitutional experimentation, and coercive rule.