Spanish invasion of Portugal ( 1762 ) The Spanish invasion of Portugal , between 9 May and 24 November 1762 , was the principal military campaign of the Spanish – Portuguese War , 1761 – 1763 , which in turn was part of the larger Seven Years ' War . It initially involved the armies of Spain and Portugal , before the French and British intervened in the conflict on the side of their respective allies . Background . Spain and Portugal had both remained neutral in the Seven Years ' War which had been officially declared in 1756 . Under Ferdinand VI Spain had enjoyed good relations with the British , and so did not join with their traditional allies France against the British . This changed with the succession of a new monarch , Charles III whose government switched to a more pro - French policy and in late 1761 the two states went to war . Prelude . The original Spanish plan was to take Almeida and then to advance towards the Alentejo and Lisbon , but after the Marquis of Sarria had been appointed commander - in - chief he decided to begin by an attack in the north with Porto as its aim . This would deal a hard blow to the British , who had large commercial interests in Porto , and would also be agreeable to Elisabeth of Parma , the Queen Mother , who was still very much a power behind the Spanish throne and wished to spare the position of her daughter Mariana Victoria , the Queen Consort of Portugal . In any case there was no point in antagonising the Portuguese unduly , and if they were not attacked in their capital they might be readier to give in . The Battle . The Spanish attack . In the beginning of May , the Spanish and French troops from Galicia crossed into Portugal and easily took the undefended towns of Chaves and Bragança and also Miranda do Douro , which was fortified but capitulated at once after an accidental explosion had made a large breach in the walls . They then overran the Trás - os - Montes plateau as far as Torre de Moncorvo , which was also an open town though the Spaniards expected it to be defended , and where they found a large number of guns and munitions . The attack on Porto came to a standstill , because the Spanish did not realize the difficulties of the country to be traversed . A further delay was caused by the fact that the main body of Spanish troops at Zamora , intended for Almeida , were held up by the flooded River Esla , a northern tributary of the Douro , which they could not cross until a pontoon - bridge had been laid across it . France also sent some 12 , 000 men , but this force was decimated by disease and never used on the field , because their relations with the Spaniards were difficult . The Marquis of Sarria lost precious time and was replaced by the Count of Aranda by mid August . British entry into the war . This delay gave the British the time to bring over five regiments under John Campbell , 4th Earl of Loudoun from the occupied island of Belle - Ile off the coast of France . They arrived in Lisbon in the third week of July . It was decided that the Earl of Loudoun would be second - in - command and that the German Wilhelm , Count of Schaumburg - Lippe would take over the command from the sick Baron Tyrawley . Siege of Almeida . In mid - August the new Spanish commander - in - chief , Conde de Aranda , crossed the Côa river , occupied Castelo Rodrigo and besieged and took the key border fortress of Almeida on August 25 . Lippe had decided he would relieve the pressure on Almeida and Porto by organizing a counter - attack . A task force under John Burgoyne supported by a sizable body of Portuguese infantry crossed the Tagus and on August 27 took the Spanish city of Valencia de Alcántara by surprise . They cleared the neighbourhood of the Spanish troops , taking a number of prisoners including a Spanish general and returned with a ransom of a year ' s taxes paid in corn . This victory raised Portuguese morale and Burgoyne was given a large diamond and the Spanish Colours captured . Lippe then organized a defense along the Zêzere River , and spent the summer studying the Spanish positions and thinking up counter - measures , directing his troops in many marches and counter - marches . He prevented the Spaniards from crossing the Tagus at Vila Velha de Ródão , and on October 7 Burgoyne defeated the Spanish at the Battle of Vila Velha and put out of action a battery of guns which were about to be emplaced , and safely crossed the river again . By mid - October the rains had swelled the Zêzere , rendering it impracticable . In November the Spanish attacked two small places , Marvão and Ouguela , but both cities were successfully held . On November 24 the Count of Aranda informed Lippe that peace preliminaries had been signed , emissaries were exchanged and an armistice agreed . Aftermath . The British and Portuguese had success by adroit marches and counter - marches , so that the Spaniards , although hugely superior in numbers , were always confronted by defenders in a good position and never dared to risk an all - out attack . There were few deaths in battle , but many soldiers on both sides died from sickness . A number of British and Spanish soldiers later to become notable in the American War of Independence served in this campaign . It has largely been ignored from public attention , perhaps because no major battles took place , it was overshadowed by other theatres of the war , and because it took place so close to the conclusion of the Seven Years War . The conflict is portrayed in the novel " Absolute Honour " by C . C . Humphreys , where the hero Jack Absolute serves with the British Light Dragoons under the command of Colonel John Burgoyne , later to be famous for his entrapment at Saratoga .