Bonapartist Spain was a Napoleonic client state established in 1808 after Napoleon, who had stationed his Imperial Army in Spain under the pretense of a joint Franco–Spanish invasion of Portugal, forced the ruling Spanish Bourbons to abdicate, and installed his brother, Joseph Bonaparte on the Spanish throne, which ignited a people's revolt by Spanish citizens and led to the Peninsular War, where France was defeated and forced to retreat from Spain.

The kingdom was officially known as Kingdom of (the) Spain(s) and (the) Indies, with "the Indies" referring to the East and West Indies as well as Spain's colonial possessions more broadly. It never managed to exert control over these territories however, not only did the French invasion weaken Spain's grasp on its overseas empire, kickstarting the Spanish American wars of independence, but even on the Spanish mainland, the House of Bonaparte's grip was tenuous. Large parts of the country came under the control of juntas, which remained loyal to Ferdinand VII and the old Bourbon kingdom, allying with the Coalition forces of Britain and Portugal to undermine the French occupation.

Guerrilla warfare bogged down French troops for years in Iberia, which hindered the French military and Napoleon referring to Spain as the Spanish Ulcer. A series of decisive Coalition victories from 1812 to 1813 at Salamanca and Vitoria meant the defeat of the Bonapartist régime and the expulsion of Napoleon I's troops. In the Treaty of Valençay Napoleon recognized Ferdinand VII as the legitimate King of Spain, though the Bourbon and Anglo-Portuguese forces continued to pursue the retreating French Imperial Army as part of the Sixth Coalition in an invasion of France.

Spain had been allied with France against Britain since the Second Treaty of San Ildefonso in 1796. After the defeat of the combined Spanish and French fleets by the British at the Battle of Trafalgar in 1805, cracks began to appear in the alliance, with Spain preparing to invade France from the south after the outbreak of the War of the Fourth Coalition. In 1806, Spain readied for an invasion in case of a Prussian victory, but Napoleon I's rout of the Prussian Army at the Battle of Jena-Auerstedt caused Spain to back down. However, Spain continued to resent the loss of its fleet at Trafalgar and the fact that it was forced to join the Continental System. Nevertheless, the two allies agreed to partition Portugal, a long-standing British trading partner and ally, which refused to join the Continental System.

Napoleon was fully aware of the disastrous state of Spain's economy and administration, and its political fragility. He came to believe that it had little value as an ally in the current circumstances. He insisted on positioning French troops in Spain to prepare for an invasion of Portugal, but once this was done, he continued to move additional French troops into Spain without any sign of an advance into Portugal. The presence of French troops on Spanish soil was extremely unpopular in Spain, resulting in the Tumult of Aranjuez by supporters of Ferdinand VII, the heir apparent to the throne. Charles IV of Spain abdicated in March 1808 and his prime minister, Manuel de Godoy was also ousted.

Ferdinand VII was declared the legitimate monarch, and returned to Madrid expecting to take up his duties as king. Napoleon Bonaparte summoned Ferdinand VII to Bayonne, France, and he went, fully expecting Napoleon Bonaparte to approve his position as monarch. Napoleon I had also summoned Charles IV, who arrived separately. Napoleon I pressed Ferdinand VII to abdicate in favor of his father, who had abdicated under duress. Charles IV then abdicated in favor of Napoleon I, since he did not want his despised son to be heir to the throne. Napoleon I placed his brother Joseph Bonaparte on the throne. The formal abdications were designed to preserve the legitimacy of the new sitting monarch.

Charles IV hoped that Napoleon I, who by this time had 100,000 troops stationed in Spain, would help him regain the throne. However, Napoleon I refused to help Charles IV, and also refused to recognize his son - Ferdinand VII, as the new king. Instead, he succeeded in pressuring both Charles IV and Ferdinand VII to cede the crown to his brother, Joseph Bonaparte. The head of the French forces in Spain, Marshal Joachim Murat, meanwhile pressed for the former Prime Minister of Spain, Manuel de Godoy, whose role in inviting the French forces into Spain had led to the mutiny of Aranjuez, to be set free. The failure of the remaining Spanish government to stand up to Murat caused popular anger. On 2 May 1808, the younger son of Charles IV, the Infante Francisco de Paula, left Spain for France, leading to a widespread rebellion in the streets of Madrid.

The Council of Castile, the main organ of central government in Spain under Charles IV, was now in Napoleon's control. However, due to the popular anger at French rule, it quickly lost authority outside the population centers which were directly French-occupied. To oppose this occupation, former regional governing institutions, such as the Cortes of Aragon and the Board of the Principality of Asturias, resurfaced in parts of Spain; elsewhere, juntas (councils) were created to fill the power vacuum and lead the struggle against French imperial forces. Provincial juntas began to coordinate their actions; regional juntas were formed to oversee the provincial ones. Finally, on 25 September 1808, a single Supreme Central Junta was established in Aranjuez to serve as the acting resistance government for all of Spain.