Representatives of Parliament and the Scottish Covenanters concluded the Solemn League and Covenant, which was sworn at Westminster in September 1643. Scotland agreed to send an army into England, while Parliament committed itself to religious reformation. The alliance altered the military balance against the king.

The Covenant's biblical and contractual language permeated sermons, devotional verse and political pamphlets. It also intensified disputes over worship that divided poets such as Milton from Presbyterian parliamentary allies.