In 1665, at the age of 16, Monmouth served in the English fleet under his uncle, the Duke of York in the Second Anglo-Dutch War. In June 1666, he returned to England to become captain of a troop of cavalry. On 16 September 1668 he was made colonel of the His Majesty's Own Troop of Horse Guards. He acquired Moor Park in Hertfordshire in April 1670. At the outbreak of the Third Anglo-Dutch War in 1672, a brigade of 6,000 English and Scottish troops was sent to serve as part of the French army (in return for money paid to King Charles), with Monmouth as its commander. He became Lord Lieutenant of the East Riding of Yorkshire and Governor of Kingston-upon-Hull in April 1673. In the campaign of 1673 and in particular at the Siege of Maastricht that June, Monmouth gained a considerable reputation as one of Britain's finest soldiers. He was reported to be replacing Marshal Schomberg as commander of England's Zealand Expedition, but this did not happen.