The post of Foreign Secretary was generally thought to be beyond his powers. Thomas Grenville, writing to the Marquis of Buckingham, expressed an opinion that he was only "put in ad interim until Lord Wellesley's arrival, who is expected in June". Mulgrave, however, showed himself fairly capable in debate. On 11 February 1805 he had to announce the breach with Spain, and to defend the seizure of the treasure ships at Ferrol before the declaration of war, and on 20 June to defend the coalition of 1805. He composed an ode on the victory of Trafalgar, and it was set to music by Thomas Arne. On 23 January 1806 Pitt died. On 28 January 1806 Mulgrave laid before the House of Lords copies of the treaties recently concluded with Russia and Sweden, to which Prussia and Austria had acceded, and on 4 February he explained their object. Three days later, on 7 February, he resigned, with the bulk of those who had been Pitt's friends.