The illegitimate son of Captain George Leslie of Blair Castle in Atholl, Alexander Leslie became a soldier and fought under Sir Horace Vere in the Netherlands (1605-7), then transferred to the Swedish army as an ensign in 1608. He served under Charles IX and his successor, the Protestant champion Gustavus Adolphus, the "Lion of the North". In 1626, Leslie was promoted to lieutenant-general and knighted by Gustavus. In 1628, he distinguished himself by successfully defending Stralsund against the Imperial commander Wallenstein and in 1630 he seized the island of Rügen for Sweden. He returned to Scotland to assist in recruiting and training the Scottish volunteers brought over to Gustavus by James, Marquis of Hamilton, in I631. Despite being severely wounded in the winter of 1631, Leslie fought at the battle of Lützen in November 1632, where Gustavus was killed. He continued in the Swedish service under Gustavus' successor Queen Christina and in 1636 was promoted to the rank of Field Marshal in the Swedish army.
In 1638, Leslie returned to Scotland in response to the crisis brought about by the imposition of the Laudian prayer-book and the signing of the Scottish National Covenant. As Scotland's greatest soldier, Leslie took command of the Army of the Covenant and organised its recruiting and training. Stern Presbyterian veterans who had served with Gustavus were invited to be his officers. In the Second Bishops' War (1640) Leslie easily brushed aside the King's men at the battle of Newburn, then captured the city of Newcastle and occupied Northumberland and Durham. In an attempt to win Leslie's allegiance, King Charles created him Earl of Leven and Lord Balgonie in October 1641.
Leven commanded an army of 10,000 Covenanters against Irish rebels during 1642. However, he disliked the war of attrition that prevailed in Ireland and returned to Scotland, leaving Major-General Robert Monro to command in Ulster. When the Scots signed the Solemn League and Covenant with the English Parliament, Leven led the Covenanter army that marched into England to fight against the King in January 1644. Leven's advance south was resisted by the Marquis of Newcastle. When the Marquis withdrew to York in April 1644, Leven joined forces with Lord Fairfax and the Earl of Manchester to besiege the city. Prince Rupert's march to relieve York resulted in the battle of Marston Moor (July 1644), where Leven had overall command of the Allied army. Believing the battle to be lost, however, Leven fled from the battlefield and is said to have galloped all the way to Leeds.
After Marston Moor, the Allied armies separated and Leven returned to the siege of Newcastle, which he stormed and captured in October 1644. However, he was reluctant to obey the English Parliament's orders to march south against the King because of the threat posed by the Marquis of Montrose. Having sent his cavalry under David Leslie back into Scotland against Montrose, Leven was forced to abandon the siege of Hereford in September 1645. Following Leslie's victory at Philiphaugh, Leven marched to besiege Newark. Exhausted by his years of campaigning, he left Leslie to conduct the siege early in 1646 and withdrew to Newcastle. In May 1646, King Charles surrendered to the Scottish army at Newark and was quickly moved to Newcastle. The King remained under Leven's supervision until he was handed over to the English Parliament in 1647 — with Leven constantly urging him to take the Covenant and to make peace.
Leven refused to support the Engager invasion of England in 1648, but was appointed Captain-General of all forces that might be raised for the defence of Scotland. Aged nearly seventy, he resigned effective command to David Leslie while remaining nominal commander-in-chief. He did not accompany the Scots-Royalist army that invaded England with Charles II in 1651 but was taken prisoner by English troops in Scotland. He was held prisoner in England until 1654, then retired to Fifeshire.
References:
Stuart Reid, All the King's Armies, 1998
David Stevenson, Alexander Leslie, first earl of Leven, Oxford DNB, 2004