Complex sentence: Instead of marching on London, he marched north towards Somerset, and on 14 June clashed with the Dorset Militia at Bridport with many of the militiamen deserting and joining Monmouth's army, before another skirmish on the 15th at Axminster.
Split into simpler sentences: Instead of marching on London, Monmouth marched north towards Somerset, and on 14 June clashed with the Dorset Militia at Bridport. Many of the militiamen deserted and joined Monmouth's army, before he fought another skirmish on the 15th at Axminster.