commander of the Scottish army that from 1644 to 1646 fought on the side of Parliament in the English Civil Wars between Parliament and King Charles I.

Leslie joined the Swedish army in 1605 and served brilliantly in the Thirty Years' War in central Europe. In 1628 he distinguished himself by successfully defending Stralsund against the imperial commander Wallenstein, and in 1636 he became a field marshal under the Swedish king Gustavus II Adolphus.

By the time he returned to Scotland in 1637, the country was in turmoil over King Charles I's attempts to impose Anglican forms of worship on the Presbyterian Church of Scotland. Leslie readily pledged to defend the Presbyterian religion and indeed had encouraged Scottish troops on the European continent to do so. During the nearly bloodless First and Second Bishops' Wars (1639–41) between England and Scotland, he commanded the Scottish army. He occupied northeastern England in August 1640, remaining there until the war's end. In a fruitless attempt to win his allegiance, Charles then made him Earl of Leven and Lord Balgonie (October 1641).

Leven led Scottish troops against Roman Catholic rebels in Ireland in 1642–43, but he returned to Scotland (January 1644) to take charge of the Scottish army that entered England to fight for Parliament. He played a leading role in the campaigns of 1644–45, and in May 1646 Charles I surrendered to him at Newark, Nottinghamshire. After handing the king over to Parliament (January 1647), Leven returned with his army to Scotland and retired from active service. He was powerless to prevent the Scottish Royalists from sending troops into England in 1648, but the execution of Charles I by the Independents (radical Puritans) brought him into the Royalist camp of King Charles II. In 1650–51 the aged general commanded the forces that defended Scotland from the invading army of Oliver Cromwell. Captured by English dragoons at Alyth in August 1651, Leven was confined until 1654, when he once more retired.