Promoted Captain in October 1660 by the Frisian stadtholder William Frederick, Prince of Nassau-Dietz, Van Coehoorn first saw action in 1665 during the Second Anglo-Dutch War when he helped repulse an English-funded invasion by the Bishop of Munster. During the Franco-Dutch War of 1672-78, he was wounded at the unsuccessful defense of Maastricht in 1673, then fought at Grave and Seneffe in 1674. He was promoted Major by Henry Casimir II, Prince of Nassau-Dietz, took part in the battles of Cassel and Saint-Denis and ended the war as a Lieutenant-Colonel.. How many years after his promotion did he first see conflict?