In April 1806 Lieut.-Colonel Pack proceeded, with the first battalion of the SEVENTY-FIRST, in the expedition to South America under the command of Brigadier-General William Carr Beresford, afterwards General Viscount Beresford, and was present in six actions with the enemy in that country, and was wounded, and detained a prisoner, contrary to the capitulation which restored the town of Buenos Ayres to the Spaniards. Lieut.-Colonel Pack subsequently effected his escape with Brigadier-General Beresford, and joined the army at Monte Video, under Brigadier-General Sir Samuel Auchmuty, who, at the request of Lieut.-Colonel Pack, directed a board of naval and military officers to inquire into the particulars of his escape, by whom it was unanimously approved, and he was declared free to serve.[34] Lieut.-Colonel Pack was then detached with a small force to Colonia, where he commanded successfully in two actions; namely, in an attack on the enemy on his post, and in one made on his, at St. Pedro, when, after a forced night march, the troops under his orders, amounting to 1,013 rank and file, routed the enemy, on the 7th of June 1807, and captured a standard, together with 105 prisoners, including one lieut.-colonel and five other officers; all his artillery, baggage, &c. were likewise taken. 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.