After the solemnization of his marriage, he passed some time at Rome, where he accepted of a blue garter, affected to appear with the title of duke of Northumberland, and for awhile enjoyed the confidence of the exiled Prince. But as he could not always keep himself within the bounds of the Italian gravity, and having no employment to amuse his active temper, he ran into his usual excesses, which giving offence, it was thought proper for him to remove from that city for the present, lest he should fall into actual disgrace. Accordingly the duke quitted Rome, and went by sea to Barcelona, where hearing that the trenches were opening before Gibraltar, he resolved upon a new scene of life, which few suspected he would ever engage in. He wrote a letter to the King of Spain, acquainting him, ’That he designed to take up arms in his Majesty’s service, and apprehending that his forces were going to reduce the town of Gibraltar under his obedience, he hoped he should have his permission to assist at the siege as a volunteer.’
This done, he went to the camp, taking his duchess along with him, and was received with all the marks of respect due to his quality. The Conde de la Torres, who commanded there, delivered him an obliging letter from the King his master, thanking him for the honour he intended him, by serving in his troops, and during that siege, appointed him his aid-de-camp, by which, post the duke was to give an account of all transactions to his Majesty himself, which obliged him to be often in the trenches, and to expose his person to imminent danger. During this siege want of courage was never imputed to him; on the contrary, he was often guilty of the most imprudent rashness. One evening he went close to the walls, near one of the posts of the town, and threatened the soldiers of the garrison. They asked who he was? he readily answered, the duke of Wharton; and though he appeared there as an enemy, they suffered him to return to the trenches without firing one shot at him.
This siege was ended, and the duke received no other hurt, than a wound in his foot by the bursting of a grenade, and when nothing more was to be done in the camp, he went to court, where he was held in the utmost respect by the principal nobility. The King likewise, as a mark of his favour, was pleased to give him a commission of Colonel Agregate (that was the term) to one of the Irish regiments, called Hibernia, and commanded by the marquis de Castelar.