OTL; 1808; Sir Arthur Wellesley, who latter history would remember as Duke of Wellington, gathered a fleet an 10.000 men in Cork, Ireland, ready to attack the city of Buenos Aires for a third time, as in the previous two times it was failed to be captured permanently. This time, Wellington was planning to ally with the creoles against the Spaniards, so he would arrive Rio de la Plata not as an invader, but a liberator. His further plan was to help Francisco de Miranda in making his dream true: liberating Spanish America and create an empire.
Suddenly, the whole set of alliances changed: Napoleon invaded Spain, starting the Peninsula War, so the British became allies of Spain, and their obligation as such was to stop any attack on their colonies and help them throwing the invader out of their homeland.
This is the history of what would had happened if the British decided it was too late to change allies or, no matter the alliances, the Spanish colonies ahd to be invaded in order to secure such a potential region of commercial influence, and proceed with the Third Invasion to Rio de la Plata.
Tercera Invasión Inglesa: La Conquista (Third English Invasion: The Conquest)
It was 7th July 1808 when Wellesley's fleet arrived the port of Quilmes, where both artillery and cavalry could disembark safely. Viceroy Santiago de Liniers was already prepared to reject this invasion, as he did with the last one, the past year.However, he was not prepared to see commander Manuel Belgrano, Secretary of the Consulate, occupying the cabildo by force and taking him prisoner. An internal struggle began, when the Chief of the Regiment of Patricians, Cornelio Saavedra, main supporter of Liniers, tried to capture the cabildo. But his effort proved useless when he found himself out surrounded by rebel creole troops commanded by Belgrano and his cousin Juan José Castelli, and the British forces of Sir Arthur Wellesley.
By the next day, the city was completely taken by the British. Some Spaniards achieved to flee the city prior to the arrival of Wellesley. The most noticed, Martín de Álzaga, wrote a letter to his friend Francisco Javier de Elío, leader of the junta of Montevideo, who didn't recognise Liniers' government on Buenos Aires. Some days after, the Rio de la Plata was a battlefield, disputed between the British/creoles from Buenos Aires and the Spaniards/loyalists from Montevideo.
The creole generals Juan Martín de Pueyrredón and José Rondeau led the warring efforts of Buenos Aires in the Eastern Bank (Banda Oriental, OTL Uruguay), while Wellesley and the patriots met at Buenos Aires to find a way to invade the Bank and secure the Rio de la Plata under British-patriot joint control.
The Peninsula War
Althoguh the British did support the Spanish war of independence, many Spaniards called them traitors, as they were taking advantage from their situation to steal their overseas colonies. Both conservatives and liberals doubted of the intetions of the British by helping them. Between that and the apparent lack of interest of Ferdinand VII on which side won the war as far as he gets to conserve his power and influence, almost no independent Spanish army supported neither the Bourbon restoration nor the British help any more. The guerrillas divided between the French and the Spanish nationalist sides.
Due to the British unpopularity, Sir John Moore, responsible for leading the British army that is supposed to "help" the Spaniard warring efforts against the French, finds himself being objective of guerrilla tactics from the Spaniard rebels soon after winning the battle of Sahagún.
By 3 May 1809, the British cavalry led by lord Henry Peget suffered an ambush from a Spanish guerrilla squad commanded by Juan Martín Díez in the outskirts of Villaselán. Lord Peget got captured, as did his liutenant, Sir John Slade. The surviving 205 troops from the original 610 scaped to warn Sir Moore about the dangers of get to deep into guerrilla-controlled land.
Something similar happened when the Anglo-Portuguese army led by William Beresford tried to cross the border from Portugal to Galicia. They were beaten in the battle of Costelo Branco by Spanish troops led by Gregorio García de la Cuesta, and later in the battle of Belmonte, when the Spaniard attacked them while preparing for a new attempt to cross the border. Their last attempt was at Penamacor, where they managed to defeat the guerrilleros sent by de la Cuesta, but fell before the troops of Francisco Xavier Venegas in Aljucén, while marching southwards to surround Cuesta's army at Badajoz.
These defeats from the British and Portuguese armies made one thing clear: The Spanish ere not only rejecting the French invitation, but would not allow any power to invade their homeland any more.
Invasion of the Interior
Meanwhile, in the Americas, Buenos Aires' cabildo sent messages to all provincial cabildos of the viceroyalty. After some deliberations, there first one to join the Anglo-bonaerense patriot coalition was Entre Ríos, followed by Santa Fe. With the territories of the littoral secured for the patriots, Wellesley only needed to take control of the military governorships of Misiones and the yet loyalist Junta of Montevideo, in the Eastern Bank.