Lieut.-Colonel Pack was shortly afterwards appointed by Lieut.-General John Whitelocke to the command of all the light companies in his army, and joined the force then in the River Plate destined to act against Buenos Ayres. He was engaged in two successful actions with the enemy prior to the unfortunate attack on the town, in which he was three times wounded. Towards the end of 1807 he returned to Europe, and early in 1808 had the SEVENTY-FIRST completely re-equipped; and, proceeding with the first battalion of the regiment from Cork to Portugal, on the 17th of June following, in the expedition under Lieut.-General Sir Arthur Wellesley, was present in the battles of Roleia and Vimiera, on the 17th and 21st of August 1808, which rescued Portugal from the French. The conduct of the battalion and of Lieut.-Colonel Pack was noticed in the public despatches, and the troops received the thanks of both Houses of Parliament.
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.