Under the command of the governor of Buenos Aires, Pedro Antonio de Cevallos, the expedition captured the Portuguese settlement of Colónia do Sacramento and fought off a joint Anglo-Portuguese attempt to recapture it.

Having been notified months in advance that Spain was planning to enter the conflict and declare war on Portugal, Cevallos launched an expedition against Colónia do Sacramento on 3 September 1762, sailing at the head of an expeditionary fleet from Buenos Aires which carried 2,700 soldiers.

From January to April 1763, Cevallos launched several attacks on Portuguese outposts in the region, capturing the forts of Santa Tereza and San Miguel and occupying the village of Rio Grande.

The Portuguese refused to accept losing control over these outposts, launching several incursions into Spanish-held territory; these actions eventually resulted in an undeclared war breaking out between Spain and Portugal between 1776 and 1777.

[3] Meanwhile, the Portuguese garrison at Colónia do Sacramento, which consisted of 400 regular infantrymen, 40 cavalrymen, 32 artillery gunners, 230 militia, and the crew of ten small vessels in the port harbour, was reinforced by a 10-ship convoy from Rio de Janeiro on 24 September.

Spanish casualties during the siege amounted to 12 killed and 200 wounded, while their prizes of war included 87 artillery pieces and 26 ships anchored in the Colónia do Sacramento roadstead, mostly British merchantmen.

During a stopover at Lisbon, the expedition was joined by a Portuguese fleet of 2 troopships and 5 store ships under the command of lieutenant colonel Vasco Fernandes Pinto Alpoim carrying 500 soldiers.

The expedition, which was hoping to attack Spanish settlements on the River Plate before the colonial authorities there were informed of the outbreak of war between Spain and Portugal, had set sail for Brazil on 3 August 1762.

After a three-hour bombardment of the city walls, counter-fire from the Spanish defenders caused a fire on the Lord Clive, eventually spreading to the powder magazine, which exploded, destroying the ship and killing 272 crew members, including McNamara.

These offensives resulted in Portuguese governor Eloy Madureira ordering the village of Rio Grande to be abandoned in the face of the advancing Spaniards, who occupied the settlement on 24 April.

[6] Though the expedition did not influence the terms of the Treaty of Paris, the Spanish successes in South America contrasted with their failures in the other theatres of war, including a failed invasion of Portugal and major defeats at Havana and Manila.

[8] Colónia do Sacramento was returned to Portugal as part of the terms of the Treaty of Paris, while Santa Tereza (which had been extensively refurbished on Cevallos' orders[9]), San Miguel and Rio Grande remained under Spanish control.