Complex sentence: Between the two Bishops' Wars the Covenanters maintained one regiment of infantry and many of their officers on half pay who had drilled the local militias, now armed with firearms purchased in the Netherlands.
Split into simpler sentences: Between the two Bishops' Wars the Covenanters maintained one regiment of infantry and many of their officers who had drilled the local militias on half pay. The militas were now armed with firearms purchased in the Netherlands.