Like many others, Holles had been reluctant to go to war, which divided his family; his cousin Gervase Holles served in the Royalist army, while another relative, William Holles, was killed outside Oxford in March 1644. Although not present at Brentford, contemporaries suggested the casualties suffered by his regiment was one reason Holles became a leading member of the Parliamentarian 'Peace Party'. This caused a breach with Pym, who believed Charles had to be defeated militarily, since he would never voluntarily keep commitments he considered forced on him.