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Why did Parliament form an alliance with the Scottish Covenanters in 1643?

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Painting of the Battle of Naseby shows cavalry and infantry clashing, with soldiers on horseback leading a charge and fallen fighters on the ground amid the chaos of battle. — English Civil Wars *Battle of Naseby*, by an unknown artist. The victory of the Parliamentarian New Model Army, under Sir Thomas Fairfax and Oliver Cromwell, over the Royalist army, commanded by Prince Rupert, at the Battle of Naseby (June 14, 1645) marked the decisive turning point in the English Civil War.

In 1643, the English Parliament formed an alliance with the Scottish Covenanters primarily to secure essential military support during a period of significant Royalist momentum. By the spring and early summer of 1643, Royalist forces had achieved several successes, and the prospect of the king receiving additional aid from Ireland threatened the Parliamentarian cause.1

To counter these developments, the two parties signed the Solemn League and Covenant on September 25, 1643. This agreement was driven by distinct motivations:12

Parliamentarians: They viewed the alliance as a necessary civil and military pact to obtain the reinforcements needed to win the war.2
Scottish Covenanters: They were desperate to protect their own religious revolution at home and insisted upon the establishment of Presbyterianism in England as a condition of their support.13

In exchange for this religious commitment, the Scots agreed to send an army of 21,000 men to serve in England. This intervention proved critical to the Parliamentarian war effort, most notably at the Battle of Marston Moor in 1644.1