In the American Revolutionary War of 1775 to 1783, Admiral Keppel's resignation during the crisis following the Battle of Ushant in 1778 left a vacancy for command of the Channel Fleet. On 23 January 1778 Darby became a rear-admiral and on 19 March 1779 he was promoted to vice-admiral, thanks to his association with Lord Sandwich, First Lord of the Admiralty, during the court martial of Admiral Palliser. Thus he unexpectedly came to command the Channel Fleet in 1780 at a time of grave danger for the kingdom.