The Engagers were a faction of the Scottish Covenanters, who made "The Engagement" with King Charles I in December 1647 while he was imprisoned in Carisbrooke Castle by the English Parliamenterians after his defeat in the First Civil War.
The Covenanters, a national Presbyterian movement, governed Scotland from 1639-51, during the civil Wars of the Three Kingdoms. They supported the English Parliamentarians in the First English Civil War in return for the Long Parliament agreeing to the Solemn League and Covenant, which promised reform in England and support for the Scottish church settlement.
However, after the English Parliament's victory, they feared being sidelined - especially by the more radical Independent faction that had a strong following in the New Model Army both in the rank and file and among the Grandees like Oliver Cromwell.
As a result, some of their General Assembly members signed a secret treaty with Charles I in 1647, known as The Engagement, which promised that Charles would support the establishment of Presbyterianism, in England for a period of three years, in return for a military alliance with the Covenanters. Not all Covenanters agreed with The Engagement and a large faction known as the Kirk party, strongly influenced by Archibald Campbell, 1st Marquess of Argyll were opposed, because King Charles refused to take the Covenant personally and they feared he would not honour the agreement if he were restored to power. However the faction that supported The Engagement, known as The Engagers, outmaneuvered the Kirk party, defeating some of them in a skirmish at Mauchline Muir, and organised an expeditionary army. Neither of the more experienced Scottish Generals, Lord Leven or David Leslie, was willing to lead the army as they sided with Argyll, so the command was given to the less experienced Duke of Hamilton.
The army was sent to England to try to enforce this deal but Cromwell routed it at the battle of Preston in 1648, and Charles I was executed in 1649. This defeat discredited the Engager party. However the Engagers won a victory in the following month at the Battle of Stirling (1648). The more radical Covenanters, in the Kirk party, insisted that any future deal with the King or other would have to include the public endorsement of their demands and not the secret promise of concessions in the future. Charles II agreed to their demands in the Treaty of Breda (1650), but his defeat at the Battle of Worcester 1651 at the hands of Oliver Cromwell negated any agreements he had made with the Covenanters. Later he averred that "Presbyterianism is no religion for a gentleman".