The Protestant leaders of the embattled English parliament, faced with the threat of Irish Catholic troops joining with the Royalist army, requested the aid of the Scots. The Presbyterian Covenanters promised their aid against the 'papists', on condition that the Scottish system of church government was adopted in England. This was acceptable to the majority of the English Long Parliament, as many of them were presbyterians, while others preferred allying with the Scots to losing the Civil War.
After some haggling a document called the "Solemn League and Covenant was drawn up. This was practically a treaty between England and Scotland for the preservation of the reformed religion in Scotland, the reformation of religion in England and Ireland "according to the word of God and the example of the best reformed churches," and the extirpation of popery and prelacy. It did not explicitly mention presbyterianism, and included some ambiguous formulations which left the door open to the English Independents, another strong faction on the English Parliamentary side, particularly in the parliamentary armies. It was subscribed to by many in England, Scotland, and Ireland, approved by the English Long Parliament, and, with some slight modifications, by the Westminster Assembly of Divines. This agreement meant that the Covenanters sent another army south to England to fight on the Parliamentarian side in the First English Civil War. Not all those on the parliamentarian side were happy with this arrangement and some, like John Lilburne, choose to leave the parliamentary armies rather than take the oath prescribed in the Act enforcing the Solemn League and Covenant.
After the Royalists had lost the First Civil War, Charles I was able to enter into an "Engagement" with the majority of the Covenanters in which the Covenanters agreed to support Charles in the Second English Civil War against their mutual enemy the English Independents, in return for him imposing presbyterianism for three years on England. In 1648 the Royalists and the Covenanters were defeated at the Battle of Preston, and Charles was executed in January 1649. The Scottish Covenants persuaded the exiled Charles II of England to agree to the terms of the Solemn League and Covenant in the Treaty of Breda (1650). However the defeat of the Royalist and Scottish army at the Battle of Worcester in 1651 ended the relevance of the Solemn League and Covenant as the power of the Presbyterians was broken on both sides of the border.
After the Restoration the English Parliament passed the Treason Act 1661, which declared that the Solemn League and Covenant was unlawful, was to be abjured by all persons holding public offices, and was to be burnt by the common hangman.