The Peninsular War (1808–1814) was fought in the Iberian Peninsula by the Iberian nations Spain and Portugal, and the United Kingdom against the invading and occupying forces of the First French Empire during the Napoleonic Wars. In Spain, it is considered to overlap with the Spanish War of Independence. It overlapped with the War of the Fifth Coalition (1809) and the War of the Sixth Coalition (1812-1814).

The war can be said to have started when the French and Spanish armies invaded and occupied Portugal in 1807 by transiting through Spain, but it escalated in 1808 after Napoleonic France occupied Spain, which had been its ally. Napoleon Bonaparte forced the abdications of Ferdinand VII and his father Charles IV and then installed his brother Joseph Bonaparte on the Spanish throne and promulgated the Bayonne Constitution. Most Spaniards rejected French rule and fought a bloody war to oust them. The war on the peninsula lasted until the Sixth Coalition defeated Napoleon in 1814, and is regarded as one of the first wars of national liberation. It is also significant for the emergence of large-scale guerrilla warfare.

In 1808, the Spanish army in Andalusia defeated the French at the Battle of Bailén, considered the first open-field defeat of the Napoleonic army on a European battlefield. Besieged by 70,000 French troops, a reconstituted national government, the Cortes—in effect a government-in-exile—fortified itself in the secure port of Cádiz in 1810. The British army, under Arthur Wellesley, the future Duke of Wellington, guarded Portugal and campaigned against the French alongside the reformed Portuguese Army and provided whatever supplies they could get to the Spanish, while the Spanish armies and guerrillas tied down vast numbers of Napoleon's troops. In 1812, when Napoleon set out with a massive army on what proved to be a disastrous French invasion of Russia, a combined allied army defeated the French at Salamanca and took the capital Madrid. In the following year the Coalition defeated King Joseph Bonaparte's army at the Battle of Vitoria, paving the way for victory in the war in the Iberian Peninsula.

Pursued by the armies of Britain, Spain and Portugal, Marshal Jean-de-Dieu Soult, no longer getting sufficient support from a depleted France, led the exhausted and demoralized French forces in a fighting withdrawal across the Pyrenees during the winter of 1813–1814. The years of fighting in Spain were a heavy burden on France's Imperial Army. While the French won several battles they were eventually defeated, as their communications and supplies were severely tested and their units were frequently isolated, harassed or overwhelmed by Spanish partisans fighting an intense guerrilla war of raids and ambushes. The Spanish armies were repeatedly beaten and driven to the peripheries, but they would regroup and relentlessly hound and demoralize the French troops. This drain on French resources led Napoleon, who had unwittingly provoked a total war, to call the conflict the "Spanish ulcer".

For France, the Peninsular War bogged down Napoleon's troops, which allowed the rest of Europe to challenge Napoleon once more, including in the War of the Fifth Coalition, the French invasion of Russia, and culminating in Napoleon's defeat in the War of the Sixth Coalition. The war against Napoleon's occupation led to the Spanish Constitution of 1812, promulgated by the Cortes of Cádiz, later a cornerstone of European liberalism. Though victorious in war—France would never again threaten a full-scale invasion of Spain—the burden of war destroyed the social and economic fabric of both Portugal and Spain, and the following civil wars between liberal and absolutist factions ushered in revolts in Spanish America and the beginning of an era of social turbulence, increased political instability, and economic stagnation.

The Treaties of Tilsit, negotiated during a meeting in July 1807 between Emperors Alexander I of Russia and Napoleon, concluded the War of the Fourth Coalition. With Prussia shattered, and the Russian Empire allied with the First French Empire, Napoleon expressed irritation that Portugal was open to trade with Britain. Pretexts were plentiful; Portugal was Britain's oldest ally in Europe, Britain was finding new opportunities for trade with Portugal's colony in Brazil, the Royal Navy used Lisbon's port in its operations against France, and he wanted to deny the British the use of the Portuguese fleet. Furthermore, Prince John of Braganza, regent for his insane mother Queen Maria I, had declined to join the emperor's Continental System against British trade.

Events moved rapidly. The Emperor sent orders on 19 July 1807 to his Foreign Minister, Charles Maurice de Talleyrand-Périgord, to order Portugal to declare war on Britain, close its ports to British ships, detain British subjects on a provisional basis and sequester their goods. After a few days, a large force started concentrating at Bayonne. Meanwhile, the Portuguese government's resolve was stiffening, and shortly afterward Napoleon was once again told that Portugal would not go beyond its original agreements. Napoleon now had all the pretext that he needed, while his force, the First Corps of Observation of the Gironde with divisional general Jean-Andoche Junot in command, was prepared to march on Lisbon. After he received the Portuguese answer, he ordered Junot's corps to cross the frontier into the Spanish Empire.

While all this was going on, the secret Treaty of Fontainebleau had been signed between France and Spain. The document was drawn up by Napoleon's marshal of the palace Géraud Duroc and Eugenio Izquierdo, an agent for Manuel Godoy. The treaty proposed to carve up Portugal into three entities. Porto and the northern part was to become the Kingdom of Northern Lusitania, under Charles II, Duke of Parma. The southern portion, as the Principality of the Algarves, would fall to Godoy. The rump of the country, centered on Lisbon, was to be administered by the French. According to the Treaty of Fontainebleau, Junot's invasion force was to be supported by 25,500 Spanish troops. On 12 October, Junot's corps began crossing the Bidasoa River into Spain at Irun. Junot was selected because he had served as ambassador to Portugal in 1805. He was known as a good fighter and an active officer, although he had never exercised independent command.