Philip entered the military profession at an early age and in July 1620 volunteered to join Sir Horace Vere's expedition to aid Frederick V of Bohemia in the Electorate of the Palatinate. He served in it until the Bohemian defeat in 1623, participating in the two sieges of Frankenthal (where he was married in 1622). He then went on to serve Maurice of Nassau in the Netherlands, receiving a commission in 1625. At the sieges of Breda in 1625 and 1637 he was wounded, and under his old commander, Lord Vere, he was present when 's-Hertogenbosch and Maastricht were attacked in 1629. By 1632 he was a sergeant major and led many of the sorties at Maastricht with distinction. He also became deeply interested in religion, writing small private religious volumes for his family.