The Engagement

'When this treaty (which was called the Engagement, because the Commissioners engaged to restore the King by force of arms) was presented to the Scottish Parliament, it was approved by the more moderate part of the Presbyterians, who were led by the Duke of Hamilton, together with his brother, the Earl of Lanark, the Lord Chancellor Loudon, and the Earl of Lauderdale; this last being destined to make a remarkable figure in the next reign.  But the majority of the Presbyterian clergy, headed by the more zealous of their hearers, declared that the concessions of the King were entirely insufficient to engage Scotland in a new war, as affording no adequate cause for a quarrel with England.  This party was headed by the Duke of Argyle...'

The Engagement between Charles I of England and Covenanters was agreed to on December 27, 1647 (per Rampant Scotland).  Charles was confined in Carisbrooke Castle (wikipedia image) at the time.  Under the terms of this agreement, Scotland agreed to invade England to restore Charles to his throne, while Charles conceded the right to practice Presbyterianism for three years.  Sir Walter Scott covered this history in his "Tales of a Grandfather" (text above).