Peninsular War, phase of the Napoleonic Wars fought in the Iberian Peninsula by Great Britain, Portugal, and Spanish guerrillas against Napoleonic France from 1808 to 1814. In 1807 Spain agreed to support Napoleon in a war with Portugal. French troops marched through Spain and easily subdued Portugal, but occupied key Spanish cities in the process. After a palace revolution that led to the abdication of Charles IV of Spain and the naming of his son Ferdinand VII as his successor, Napoleon forced both to abdicate (May 1808) and had his brother Joseph Bonaparte proclaimed king of Spain.

The Spanish people rose in revolt, and the Portuguese quickly followed suit. After several French defeats, a British expeditionary force under Arthur Wellesley (later duke of Wellington) landed (August 1808) in Portugal and defeated the French army of General Andoche Junotat Vimeiro. A British force then invaded Spain, beginning a long series of seesaw battles.

Napoleon himself hastened to Spain and restored the French position, occupying Madrid and Zaragoza, which surrendered to the French after a bloody 2-month siege. By the spring of 1809 French forces controlled most of Spain but could not pacify the country or drive the British out of the peninsula.

The inefficient Spanish regular army contributed little to the fighting and soon dissolved altogether, but the Spanish people formed guerrilla bands that pinned down French garrisons, intercepted dispatches, and attacked convoys. The main battles—at Talavera de la Reina (1809), Salamanca (1812), and Vitoria (1813)—were fought between French and British forces. In the summer of 1813 British, Spanish, and Portuguese troops drove the French out of Spain, thereby putting an end to the Peninsular War.

The Spanish guerrillas turned the Peninsular War into what Napoleon described as his “Spanish ulcer.” The constant demand for men and money to continue the war in Spain greatly weakened France and ultimately contributed to Napoleon's final defeat in Europe.