Field Marshal Charles Lennox, 3rd Duke of Richmond, 3rd Duke of Lennox, 3rd Duke of Aubigny,, styled Earl of March until 1750, of Goodwood House in Sussex and of Richmond House in London, was a British Army officer and politician. He associated with the Rockingham Whigs and rose to hold the post of Southern Secretary for a brief period. He was noteworthy for his support for the colonists during the American Revolutionary War, his support for a policy of concession in Ireland and his advanced views on the issue of parliamentary reform. He is believed by many to be the source of the second parchment copy of the US Declaration of Independence, known as the 'Sussex Declaration'. He went on to be a reforming Master-General of the Ordnance first in the Rockingham ministry and then in the ministry of William Pitt the Younger.

Field marshal (FM) has been the highest rank in the British Army since 1736. A five-star rank with NATO code OF-10, it is equivalent to an Admiral of the Fleet in the Royal Navy or a Marshal of the Royal Air Force in the Royal Air Force (RAF). A Field Marshal's insignia consists of two crossed batons surrounded by yellow leaves below the Tudor Crown. Like Marshals of the Royal Air Force and Admirals of the Fleet, Field Marshals traditionally remain officers for life, though on half-pay when not in an appointment or retired. The rank has been used sporadically throughout its history, and was vacant during parts of the 18th and 19th centuries. After the Second World War, it became standard practice to appoint the Chief of the Imperial General Staff to the rank on his last day in the post. Army officers occupying the post of Chief of the Defence Staff, the professional head of all the British Armed Forces, were usually promoted to the rank upon their appointment.