Lieut.-Colonel Pack afterwards marched into Spain, under Lieut.-General Sir John Moore, and was at the affair of Lugo on the 5th of January 1809, and at the battle of Corunna on the 16th of that month, after which he returned to England, and embarked in July following for Holland, under Lieut.-General the Earl of Chatham. On landing at Walcheren, Lieut.-Colonel Pack was appointed to command a small corps of cavalry and light infantry; was employed in the siege of Flushing, and particularly named by Lieut.-General Sir Eyre Coote for the command of a detachment to storm an advanced work on the right of the enemy’s line. These orders were successfully executed, the detachment taking forty-nine prisoners, and spiking the guns, though defended by five times the number of men under Lieut.-Colonel Pack. After the surrender of Flushing he was appointed commandant of Ter Veer, where he was dangerously ill for a short period, but remained until the island was evacuated, on which occasion, in conjunction with Commodore Owen, he commanded the rear-guard of the army.
Soon after the return of the Seventy-first to England, in December 1809, the battalion was prepared again for active service; but the government did not consider the men had sufficiently recovered from the effects of the Walcheren fever.
Lieut.-Colonel Pack, being extremely anxious to bear a part in the momentous campaign about to commence in the Peninsula, obtained His Majesty’s permission to proceed to Portugal, and offer his services to Viscount Wellington and Marshal Beresford. Both generals having decided that he could not be more usefully employed than with the Portuguese troops, he accepted an infantry brigade in that service, and took the command of it just before the siege of Ciudad Rodrigo by Marshal Massena, previously to his invasion of Portugal.
On the 25th of July 1810 Lieut.-Colonel Pack was appointed aide-de-camp to the king, with the rank of colonel in the army. After the surrender of Ciudad Rodrigo, of Almeida, and Marshal Massena’s passage of the Coa, Colonel Pack’s brigade (an independent one) was directed to take a separate route with a regiment of cavalry attached to it, and remained in presence of the enemy’s army at St. Combadoa, retiring slowly before it, on his advance to the position at Busaco. The conduct of the brigade was noticed in that battle, which was fought on the 27th of September 1810. In the admirable retreat afterwards to the lines of Lisbon, it formed, with the light division and cavalry, the rear-guard of the allied army. The first battalion of the Seventy-first having at that period joined Viscount Wellington, Colonel Pack’s wish was to have returned to the battalion, but by the desire of both commanders-in-chief, he continued to serve in the Portuguese army.
In 1811 the brigade was in the advance guard in following the enemy up to his position at Santarem; was at the out-posts there, and again in the advance on the further retreat of the French from Portugal. It was employed in the investment of Almeida, and in the operations against Marshal Marmont, on his advance to the relief of Ciudad Rodrigo in January 1812. At the siege of Ciudad Rodrigo it bore a distinguished part. It marched to the siege of Badajos, and was in active operations against the enemy on his advance to the Tagus, and subsequent retreat from Portugal. It moved in the advanced guard on the march of the allies to Salamanca and the Douro. It suffered severely in the battle of Salamanca on the 22d of July 1812.
The brigade was in the march to and capture of Madrid; in the march to Burgos, and subsequent siege of that place. Previously to the siege of Burgos, detachments under Colonel Pack’s command carried by assault the horn-work of that castle, after a desperate and gallant action, for which the special thanks of His Royal Highness the Prince Regent, and the Commander-in-Chief, were given to the troops, through the Marquis of Wellington. In the retreat from Burgos, which commenced in October 1812, the brigade under Colonel Pack formed the rear-guard, and from thence to the frontier of Portugal was very frequently in presence of the enemy.
In the memorable advance of the Marquis of Wellington into Spain, in May 1813, and the passage of the Ebro, the brigade was in the advanced guard of the left column of the army under Lieut.-General Sir Thomas Graham, afterwards Lord Lynedoch. It was in the battle of Vittoria, fought on the 21st of June 1813, and again in the advance of Lieut.-General Sir Thomas Graham’s corps, in the pursuit of the French to the Bidassoa. Shortly afterwards, Major-General Pack, to which rank he was advanced on the 4th of June 1813, was appointed to the Highland brigade in the sixth division; the division at this time for a short period was also under his command, and after a forced march he arrived in time to share in the victory gained by the Marquis of Wellington over the French under Marshal Soult near Pampeluna, on the 30th of July 1813, in which action Major-General Pack was severely wounded in the head. He commanded the Highland brigade in the passage of the Bidassoa, and advance of the British into France; in the overthrow of the enemy in his fortified lines before Bayonne; the advance to and passage of the Nive; the repulse of the enemy’s attack on the British position before St. Jean de Luz; and, though not actually engaged, he was present at the signal defeat of the enemy’s desperate attack on Lieut.-General Sir Rowland Hill’s corps on the 13th of December 1813. Major-General Pack was also in the passage of the Bidassoa, the Gave D’Oleron, and the Pau; at the battle of Orthes on the 27th of February 1814; in the passage of the Adour at St. Seur, and at the battle of Toulouse on the 10th of April following, in which his brigade had nearly two-thirds of the officers and upwards of half the privates killed and wounded. Louis XVIII. was shortly afterwards restored to the throne of France, Napoleon retired to the island of Elba, and the Peninsular war terminated.
In 1813 Major-General Pack had been appointed a Knight Commander of the Portuguese Order of the Tower and Sword, and on the 2d of January 1815 was nominated a Knight Commander of the Order of the Bath. He received a cross and seven clasps for the following actions, at all of which he commanded troops, and was personally engaged: Roleia, Vimiera, Corunna, Busaco, siege of Ciudad Rodrigo, Salamanca, Vittoria, Pyrenees, Nivelle, Nive, Orthes, and Toulouse. Sir Denis Pack had received eight wounds, six of them rather severe ones; had been frequently struck by shot, and had several horses killed and wounded under him.