On 3 Nov. 1806 Wilson having been attached to the staff of Lord Hutchinson, then going on a special mission to the Prussian court, embarked with him at Yarmouth in the frigate Astræa, and was nearly wrecked in the Cattegat on the Anhalt shore, the guns having to be thrown overboard. He accompanied Lord Hutchinson and the king of Prussia to Memel in January 1807, and in February joined General Beningsen at the Russian headquarters of the army at Jarnova. He was present at the battle of Eylau on the 7th and 8th, and accompanied the headquarters to Heilsberg in March, and in April to Bartenstein, where on the 26th the emperor of Russia bestowed upon him the cross of St. George for his services at Eylau. Wilson took part in the campaign of June, was present at the action of the Passarge on the 5th, at the battle of Heilsberg on the 10th, and the battle of Friedland on the 14th, after which he retreated with the army to Tilsit.
On the conclusion of the peace of Tilsit he went to St. Petersburg, and thence to England with despatches, arriving on 19 Sept. On 2 Oct. he left England with a confidential communication from Canning to the emperor of Russia, arriving at St. Petersburg on the 20th. He left again on 8 Nov. with despatches from Lord Granville to Canning, containing intelligence which Wilson had himself been the first to procure, that the emperor of Russia was about to invade Swedish-Finland and declare war against England. Notwithstanding the fact that a Russian courier had preceded him by thirty-six hours (Wilson's passport having been expressly withheld to give the courier the advantage), Wilson pushed from Abo across the Gulf of Bothnia, in very bad weather, reached Stockholm before the courier, arranged that the courier should be delayed, sailed for England, landed in the Tees on the evening of the 29th, posted to London, and saw Canning in bed at four o'clock in the morning of 2 Dec. He was directed to keep quiet until Canning's orders to the naval authorities at Portsmouth had been executed; and on his return to breakfast with Canning the following morning he was complimented upon his activity, which had resulted in the seizure of the Russian frigate Sperknoi, with money to pay the Russian fleet, while a fast vessel had been despatched to Sir Sidney Smith to intercept the Russian fleet.
In 1808 Wilson was given the command of the loyal Lusitanian legion, a body raised out of Portuguese refugees in England under British officers, and in August went to Portugal as a brigadier-general in the Portuguese army. He was engaged in various encounters with the enemy in Castille and Estramadura during the retreat of the British to Coruña in 1808–9; and after the battle of Coruña on 16 Jan. 1809, acting in conjunction with the Spaniards beyond the Agueda, by a series of spirited and judicious movements, he kept open the communications with Ciudad Rodrigo and Almeida, and held the enemy in check. He had a good deal of desultory fighting, took part in the pursuit of Soult, and with the Lusitanian legion and three thousand Spaniards advanced to within nine miles of Madrid. After the battle of Talavera on 27 and 28 July Wilson found himself at Escalona, cut off by the enemy from Arzobispo; crossing the Tietar, he scrambled over the mountains, and with difficulty gained the pass of Baños on 8 Aug., as Ney's corps was approaching on its march from Placentia to the north. Wilson endeavoured to stay its advance, and defended the pass with spirit for some hours, but was eventually dislodged, and retreated to Castello Branco.
When the British army went into winter quarters, Wilson returned home, and, as the Lusitanian legion was absorbed in the new organisation of the Portuguese army, offered himself to Lord Wellesley for special service on 6 May 1810. For his services in the Peninsula he was promoted on 25 July to be colonel in the army, and appointed aide-de-camp to the king, and in 1811 received the Portuguese medal, and was made a knight-commander of the Portuguese order of the Tower and Sword. In this year Wilson published, in quarto form, ‘Brief Remarks on the Character and Composition of the Russian Army; and a Sketch of the Campaign in Poland in 1806 and 1807.’ In the autumn of 1811 his offer of service was accepted, and on 26 March 1812 he was given the local rank of brigadier-general in the British army, and accompanied Sir Robert Liston [q. v.], the newly appointed ambassador to the Porte, to Constantinople, with instructions to assist in the conduct of negotiations for peace between Turkey and Russia (see Wilson's diary of the journey in Addit. MS. 30160). He arrived at Constantinople on 1 July, and on 27 July went on a mission from Liston to the grand vizier at Shumla, to the Russian admiral Tchichagoff, commanding the Danube army corps at Bucharest, and finally to the emperor of Russia at St. Petersburg. He reached the headquarters of the Russian army under Barclay de Tolly in time to take part in the battle of Smolensk on 16 Aug., arrived in St. Petersburg on the 27th, and had an audience with
