The battle that broke the king
The Battle of Marston Moor on 2 July 1644 was the largest battle of the English Civil War and the engagement that effectively broke the power of Charles I in northern England. A combined force of English Parliamentarians and Scottish Covenanters destroyed the royalist army of Prince Rupert of the Rhine on the moor west of York. The Scottish army, fighting under the terms of the Solemn League and Covenant, played a decisive role.
Scotland's entry into the English Civil War was driven by religion, not English politics. The Scots had signed the Solemn League and Covenant with the English Parliament in 1643, promising military assistance in exchange for a guarantee that Presbyterianism would be established across Britain. An army of 21,000 Scots crossed the border in January 1644, under the command of Alexander Leslie, Earl of Leven — a veteran of the Swedish wars who was one of the most experienced soldiers in Europe.
At Marston Moor, the allied army of around 28,000 faced Prince Rupert's 18,000 royalists. The battle began late in the evening, with a surprise attack by the allied forces. On the left flank, Oliver Cromwell's cavalry routed Prince Rupert's horsemen. On the right, the Scottish infantry under the Earl of Leven was initially pushed back, and Leven himself left the field believing the battle lost. But the Scottish reserves, commanded by David Leslie, held firm and then counter-attacked. The royalist centre was surrounded and destroyed.
Marston Moor was a catastrophe for Charles I. He lost the north of England, York surrendered shortly afterward, and the strategic balance of the war shifted decisively. The Scottish contribution was vital — without the Covenanting army, Parliament could not have assembled a force large enough to challenge Rupert. Yet the Scots would receive little thanks. Cromwell took the credit, and when the war was won, he turned on his Scottish allies, invading Scotland in 1650 and imposing the very tyranny the Covenanters had fought to prevent.