alternatehistory.com

After the resumption of the Napoleonic Wars, the selected British strategy to defeat Napoleonic France was intended to divert French troops to Spain and defeat the combined French-Spanish fleets. Plans were made to defeat Napoleon with a direct assault on the French coast and straight into the Grande Armee, by encouraging the defection of Napoleonic allies and even through focusing on the Balkans. The latter options were temporarily cancelled as they were likely to end in defeat by Napoleonic troops or they were logistically impossible. Also, defeating enemy fleets could tie up Napoleonic French and allied troops for coastal defence duties.

To invade Spain, the Royal Navy would use 30 ships of the line and 30 frigates, with half of this force supporting the invasion of Cadiz along with 14 American warships. The remaining ships would bombard other major Spanish ports and defeat the remnants of the Spanish and Portuguese Navies plus any French warships caught in the Iberian Peninsula. This was supported by a corps of recruited colonists from South America, the majority of the regular British Army and the entire United States Army's active force. With the invasion to start in 1824, there was ample time for the British and Americans to prepare as the Spanish Navy was still in an inferior qualitative condition to fight the Royal Navy one to one. The resumption of the blockade on France would block the French Atlantic and Mediterranean fleets from sailing to intervene and weaken their quality, allowing the British to defeat the Napoleonic and allied fleets separately. In the landings, which were more prepared than in 1797, the British Army managed to besiege Cadiz while the Spanish Navy in Cadiz, already decimated by Trafalgar in 1809, rotten timber and a deteriorating quality, was completely destroyed in an attempt to defeat the British and American invasion forces. In the aftermath of the Siege of Cadiz [2 April to 10 May 1824], the British Army gained a foothold to continue its planned offensive into Spain proper.

To respond to the crisis, the Napoleonic Grande Armee was ordered to march to Madrid and defend Spain proper. British Invasion forces had, in fact, captured Seville from Cadiz and cut off Andalusia from Spain with an advance from Gibraltar. A third invasion force resulted in the surrender of Lisbon to the British for the second time in 17 years and would outflank the Spanish Army's defence on the Gauadalquivir River on 25 June 1824, leading to the declaration of a guerrilla war against the British by the Spanish government. As the British Army advanced on Madrid, guerrillas harassed British supply lines. The offensive against Spain was supported by the Royal Navy's Western Mediterranean Fleet off the Spanish coast, exploiting the destruction of the Spanish Navy to assist the advance to Madrid and tie up Spanish troops on coastal defence duties.

On 11 August 1824, the Grande Armee defeated the British Army off Segovia and marched south. Madrid was relieved from being under threat of British capture while the British Army imposed a scorched earth policy to defeat Spanish guerrillas and the remaining Spanish Army regulars launching a counteroffensive with French assistance to liberate their nation. British propaganda played on anti-Napoleonic and religious sentiment, but the British invasions of Spain and Portugal had secured their objectives of destroying 30 Spanish, 3 Portuguese and 5 French ships of the line and 35 frigates by the time the Iberian Peninsula was evacuated by July 1826, freeing up the Royal Navy to deal with Russia, Scandinavia, the French coast and the Balkans.

During the winter of 1824-1825, the British Army prepared a defensive line that stretched from Torre Verdas to Badajoz and planned another starting from the Portuguese-Spanish border to [now fortified] Seville. Due to scorched earth policies and harassment by the Royal Navy, the French-Spanish counteroffensive couldn't exploit the British Army's retreat and its humiliation by guerrillas. Still, the retreat did succeed in shortening the army's defensive line in Spain, helped by British naval supremacy in the area.