After a British attempt to invade the Spanish-held Viceroyalty of the Rio de la Plata failed in 1806, Manticoran ground forces were called into action to support a second British invasion force in the summer of 1807. Elements of the Manticoran 3rd Division, including the 2nd and 6th Ranger Regiments, 3rd Grenadier Brigade, and 9th Infantry Brigade, in total a force of about 10,000 men, were sent to attack Montevideo on the north side of the Rio de la Plata while the British, with 15,000 troops, concentrated their efforts against Buenos Aires on the south side of the gargantuan estuary. Upon landing, the Manticorans rapidly defeated a small Spanish force in the outskirts of the city before beginning their assault. The combined weight of Manticoran artillery rapidly breached the city walls, creating an opening that the 1st Grenadier Regiment assaulted through. With the Spanish defenders concentrating on the first breach, elements of 9th Infantry Brigade scaled the walls on the other side of the city, sealing the fate of the Spanish inside.
Against the backdrop of the Argentine overthrow of the Spanish-appointed viceroy and his replacement with the French naval officer Santiago de Liniers, who had recaptured Buenos Aires from the British the year before, the British attempted a second assault against the sprawling city. Instead of the rapid surrender seen in 1806, the Argentines fought back ferociously, making the British pay in blood for every house and street. After a week of fierce fighting, and with Argentine militia moving in from the countryside threatening to cut off their supplies, the British signed an armistice and rapidly evacuated from the south bank of the Rio de la Plata, all but abandoning the Manticorans on the north bank. Despite this setback, Manticoran forces remained in firm control of the coastal regions around Montevideo, and reinforcements soon began to arrive from Calendor. Additional Manticoran naval forces were also dispatched, which prevented an attempt by the Argentines on the south bank to send a force across the Rio de la Plata to attempt to retake Montevideo.
The 1st Grenadier Regiment was awarded the appellation “Guards” for forcing the first breach in the Spanish defenses of Montevideo.
The 4th Line Regiment was awarded the appellation “Royal” for continued valor and effectiveness against Spanish militia and infiltrators in the Banda Oriental.