In the winter of 1812/13 six troops of the Fifteenth were pulled out of the North, to proceed again on foreign service. With Lieut.Col. Grant in command, these troops embarked at Portsmouth in mid-January 1813, Landing in Lisbon in early February.
The regiment was reviewed by Lord Wellington at Almeida on the 18th May, (when the new scarlet shakos, and the officers shell bridles were used for the first time), in brigade with the Tenth and Eighteenth Hussars.
The Fifteenth were part of Sir Thomas Graham's force which crossed the Tras-os-Montes to turn the enemy on the Douro, arriving on the Esla shortly after Wellington had driven the French from Salamanca. At dawn on the 31st May, the Fifteenth approached the Almendra ford, and, with the 51st (King's Own Yorkshire) Light Infantry clinging to their stirrups, began the crossing. The river was both deep, and fast flowing; many casualties occurred. There would have been more among the Infantry if the Fifteenth had not assisted them, and, to this day, the two regiments commemorate the occasion with a sports competition on the anniversary whenever operational commitments allow.
Captain Thackwell led his squadron in advance to ascend the heights beyond the river, and precipitated the retreat of a French Heavy Dragoon piquet, capturing some prisoners in Villa Perdrices. The French retreat continued and swelled in number, pursued by two divisions of Capt. Thackwell's squadron (the remaining two divisions forming a reserve). For two miles over open country the Fifteenth were able to close with them from time to time, to kill and wound several, and to take prisoners. Having left the support at a fir grove, the two divisions continued the pursuit until, crossing a rivulet, they came upon 300 formed French Cavalry. Being two few to attack this force the divisions skirmished behind the rivulet, and then withdrew. The loss to the Fifteenth was one horse killed, and five men and three horses wounded. One Lieutenant and thirty five Dragoons remained of the French prisoners, about twenty having escaped in the Fir grove.
On the 2nd June, the Fifteenth supported the Tenth and Eighteenth in the action at Morales, during which action Colonel Grant was wounded.
In the subsequent campaign the Hussar brigade was frequently in front of the army, and the Fifteenth saw action at Burgos (12th June), Osma (18th June), and Vittoria (21st June), which earned the regiment another battle honour, and gold medals for Colonel Grant (in command of the brigade), and major Griffith (who commanded the regiment). Losses to the regiment at Vittoria were ten men and four horses killed, and Capt. Hancox, Lieut. Finch, one sergeant, forty-six rank and file, and sixteen horses wounded.
After Vittoria the Cavalry were reorganised, with colonel Grant taking command of a brigade consisting of the Thirteenth and Fourteenth Light Dragoons, the Fifteenth being brigaded with the Tenth under Maj.Gen. Lord Edward Somerset, and the Eighteenth, and First KGL Hussars being brigaded together. This structure was to remain for the remainder of the campaign in the Peninsula.
During the campaign, the Fifteenth saw further action in the blockade of Pampluna (October, 1813), Orthes (27th February 1814, for which the commanding officer of the Fifteenth, Maj. Edwin Griffith, received a gold medal), Grenade (1st March 1814, after which Sir Stapleton Cotton praised the gallant action of the officers and men of the Fifteenth, and recommended Captain Thackwell for the brevet rank of Major), St. Germier (16th March 1814, again receiving official praise from Sir Stapleton Cotton), Tarbes (20th March), Tournefeuille (26th March), St. Simon (27th March), Gagnac (April), and Toulouse (10th April 1814).
The restoration of peace at this point ended the exertions of the regiment, and, in view of the conspicuous conduct and valour of the Fifteenth, the regiment were granted Royal Authority to bear the word "Peninsula" on their appointments.
On the 1st June, the regiment began the march to Boulogne, from where they sailed to England in mid July. On the 31st of July they were assembled at Hounslow, and on the 3rd August were reviewed by the Prince Regent and the Duke of York, and were highly praised. A few days later the establishment was, once again, reduced to eight troops, and in September the regiment embarked at Liverpool, for service in Ireland.