The son of a gentry family, he entered Cambridge in 1616 but probably left the next year. Cromwell entered Parliament in 1628, standing firmly with the opposition to Charles I, and was active in the Short and Long Parliaments (1640), although not a conspicuous leader. During the first civil war (see English civil war) he rose rapidly to leadership because of his military ability and his genius for organizing and inspiring the parliamentary armies. His own regiment, the Ironsides, distinguished itself at Marston Moor (1644) and in numerous minor engagements.
In 1644 he pressed for a thorough reorganization of the parliamentary forces and was appointed (1645) second in command to Sir Thomas Fairfax (later Baron Fairfax of Cameron) in the resulting New Model Army, which defeated the king at Naseby in 1645. In the quarrel between the army and Parliament following the first civil war, Cromwell supported the sectarians in the army and approved the seizure (1647) of Charles from Parliament. However, he favored a moderate settlement with the king (as opposed to the radical proposals of the Levelers) until Charles's flight to Carisbrooke (1647) and secret dealings with the Scots caused him to lose all hope of further negotiations with the king.
In the second civil war he repelled the Scottish royalist invasion at Preston (1648). His political power was enhanced by the removal of Presbyterian leaders from Parliament in Pride's Purge (see under Pride, Thomas), and at the king's trial (1649) his was the leading voice demanding execution.
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