The Covenanters, as they were called, at first sided with Oliver Cromwell's Parliamentarians in the civil war that had erupted across the border.
The Parliamentarians had a lot of followers. 
mnli hypothesis: The Parliamentarians had a lot of followers.  premise: The Covenanters, as they were called, at first sided with Oliver Cromwell's Parliamentarians in the civil war that had erupted across the border.