国民盟約
After the parliaments of Scotland and England agreed the Solemn League and Covenant in 1643, David Leslie became a Major General under Alexander Leslie (now Earl of Leven) in the Army of the Solemn League and Covenant which was sent to fight alongside the forces of the English Parliament in 1644. He played an important role at the critical Battle of Marston Moor, west of York by leading a successful cavalry charge against the Royalist Cavaliers while Oliver Cromwell was wounded. This allowed the infantry time to regroup and eliminate the Royalist battalia led by another former colleague, Lieutenant General James King, Lord Eythin. David Leslie subsequently commanded the force that besieged Carlisle, Cumbria.