Prince Frederick, Duke of York and Albany

On 12 April 1793, Frederick was promoted to full general.[15] That year, he was sent to Flanders in command of the British contingent of Coburg's army destined for the invasion of France.[15] Frederick and his command fought in the Flanders campaign under extremely trying conditions. He won several notable engagements, such as the Siege of Valenciennes in July 1793,[16] but was defeated at the Battle of Hondschoote in September 1793.[15] In the 1794 campaign he gained a notable success at the Battle of Beaumont in April and another at the Battle of Willems in May but was defeated at the Battle of Tourcoing later that month.[15] The British army was evacuated through Bremen in April 1795.[15]

After his return to Britain, his father George III promoted him to the rank of field marshal on 18 February 1795.[15] On 3 April 1795, George appointed him effective Commander-in-Chief in succession to Lord Amherst[17] although the title was not confirmed until three years later.[18] He was also colonel of the 60th Regiment of Foot from 19 August 1797.[19]

On appointment as Commander-in-Chief he immediately declared, reflecting on the Flanders Campaign of 1793–94, "that no officer should ever be subject to the same disadvantages under which he had laboured".[17]

His second field command was with the army sent for the Anglo-Russian invasion of Holland in August 1799. On 7 September 1799, he was given the honorary title of Captain-General.[20] Sir Ralph Abercromby and Admiral Sir Charles Mitchell, in charge of the vanguard, had succeeded in capturing some Dutch warships in Den Helder. However, following the Duke's arrival with the main body of the army, a number of disasters befell the allied forces, including shortage of supplies.[21] On 17 October 1799, the Duke signed the Convention of Alkmaar, by which the allied expedition withdrew after giving up its prisoners.[21] 1799 also saw Fort Frederick in South Africa named after him.[22]

Frederick's military setbacks of 1799 were inevitable given his lack of experience as a field commander, the poor state of the British army at the time, and the conflicting military objectives of the protagonists. After this ineffectual campaign, Frederick was mocked, perhaps unfairly, in the rhyme "The Grand Old Duke of York":

The grand old Duke of York,
He had ten thousand men.
He marched them up to the top of the hill
And he marched them down again.
And when they were up, they were up.
And when they were down, they were down.
And when they were only halfway up,
They were neither up nor down.[23]

"The modern Circe or a sequel to the petticoat", caricature of Frederick's lover, Mary Anne Clarke by Isaac Cruikshank, 15 March 1809. The prince resigned as head of the British army ten days after the caricature's publication.

Frederick's experience in the Dutch campaign made a strong impression on him. That campaign, and the Flanders campaign, had demonstrated the numerous weaknesses of the British army after years of neglect. Frederick as Commander-in-Chief of the British army carried through a massive programme of reform.[1] He was the person most responsible for the reforms that created the force which served in the Peninsular War. He was also in charge of the preparations against Napoleon's planned invasion of the United Kingdom in 1803. In the opinion of Sir John Fortescue, Frederick did "more for the army than any one man has done for it in the whole of its history".[2]

In 1801 Frederick actively supported the foundation of the Royal Military College, Sandhurst, which promoted the professional, merit-based training of future commissioned officers.[21]

On 14 September 1805 he was given the honorary title of Warden of Windsor Forest.[24]

Frederick resigned as Commander-in-Chief on 25 March 1809, as the result of a scandal caused by the activities of his latest mistress, Mary Anne Clarke.[21] Clarke was accused of illicitly selling army commissions under Frederick's aegis.[21] A select committee of the House of Commons enquired into the matter. Parliament eventually acquitted Frederick of receiving bribes by 278 votes to 196. He nevertheless resigned because of the high tally against him.[21] Two years later, it was revealed that Clarke had received payment for furniture from Frederick's disgraced chief accuser, Gwyllym Wardle,[25] and the Prince Regent reappointed the exonerated Frederick as Commander-in-Chief on 29 May 1811.[26] The Duke's relationship with Mary Anne Clarke is used by Mary Anne's descendant, Daphne du Maurier, in her historical novel Mary Anne.[27]

Frederick maintained a country residence at Oatlands near Weybridge, Surrey but he was seldom there, preferring to immerse himself in his administrative work at Horse Guards (the British army's headquarters) and, after hours, in London's high life, with its gaming tables: Frederick was perpetually in debt because of his excessive gambling on cards and racehorses.[6] Following the unexpected death of his niece, Princess Charlotte of Wales, in 1817, Frederick became second in line to the throne, with a serious chance of inheriting it.[28] In 1820, he became heir presumptive with the death of his father, George III.[6]

• 16 August 1763 – 27 November 1784: His Royal Highness The Prince Frederick
• 27 November 1784 – 5 January 1827: His Royal Highness The Duke of York and Albany

His honours were as follows:[34]

• Royal Knight of the Order of the Garter, 19 June 1771[35]
• Knight Grand Cross (military) of the Order of the Bath, 2 January 1815[36]
• Knight Grand Cross of the Royal Guelphic Order, 12 August 1815[37]
• Knight of the Order of the Black Eagle of Prussia, 11 April 1814[38]
• Knight of the Order of the St-Esprit of France, 21 April 1814[39]
• Knight of the Order of St. Andrew of Russia, 9 June 1814[40]
• Knight of the Order of St. Alexander Nevsky of Russia, 9 June 1814[41]
• Knight Grand Cross of the Order of Charles III of Spain, 21 August 1814[42]
• Knight Grand Cross of the Order of Maria Theresa of Austria, 1814[39]