Why could Napoleon beat Europe’s armies but not Spain?

The short answer

• Economic war without an exit. The blockade damaged Britain: exports slumped, and 1808 and 1811–12 were years of crisis. It also deprived continental producers of markets and consumers of supplies. Smuggling spread from Lisbon to Riga. Each port that admitted British goods weakened Napoleon's system, so the commercial choices of Portugal, the Papal states and eventually Russia became problems he sought to solve through coercion or occupation. Enforcement created conflicts with the very states whose cooperation the blockade required.
• Bayonne: legitimacy fumbled. In parts of Italy and the Rhineland, the French could present their rule as liberation from foreign or feudal rulers. At Bayonne, Napoleon instead displaced the Bourbons of an intact Spanish kingdom and gave their crown to Joseph. Many Spaniards across social classes saw a seizure of their king's authority. Defeating their armies could secure ground, but it could not make them accept Joseph. Napoleon later called the Spanish affair the knot that strangled him.
• Insurgency plus sea power. France faced enemies with different strengths. Guerrillas disrupted intelligence and forage and made roads dangerous, though fortresses lay beyond their usual reach. Wellington could meet French armies in the field, but could not hold Spain alone against roughly 250,000 imperial troops. The Royal Navy could supply Lisbon and Cádiz, and the insurgents raised the cost of occupying the land between them. France had to counter these threats together.

What it changed

The veteran drain. The war in Iberia became a lasting second front. Men and horses committed or lost there could not serve elsewhere, reducing the resources available for the Russian campaign of 1812. France raised a multinational army that included allied conscripts of doubtful loyalty, but replacing experienced soldiers and horses was harder than raising numbers.

Nationalism changes sides. The French Revolution had demonstrated the force of an appeal to the nation. In Spain, that appeal helped sustain resistance to France itself. Prussian reformers, including Stein and Scharnhorst, rebuilt their state and army after defeat; by 1813, France's opponents as well as its supporters were mobilizing people through national appeals.

Cádiz writes the liberal script. The Cádiz constitution combined national sovereignty with a Catholic monarchy. For the next thirty years it offered liberals in the Mediterranean world and Latin America a model, while the political term “liberal” spread from Spanish usage. Ferdinand VII abolished the constitution on his return in 1814. Its supporters retained a grievance that helped fuel the revolutions of 1820.

An empire decapitated across the ocean. With the king held by France and a junta governing under siege in Spain, Spanish American cities faced a question of authority. Some formed juntas that claimed to conserve sovereignty for the captive monarch. A claim made in his name could also allow local governments to act for themselves, opening a path to independence even where its authors had not intended that outcome.