How a brutal guerrilla war in Spain became Napoleon’s fatal Spanish Ulcer

History · Published 29 September 2026 Swipe more like this →

In short: Napoleon Bonaparte intended to secure his Continental System by occupying Portugal and Spain, but his overreach triggered a nationalist revolt. This Peninsular War drained French resources, tied down hundreds of thousands of troops, and provided the British with a vital foothold that eventually contributed to the Emperor's downfall.

In 1807, Napoleon sought to enforce an economic blockade against Britain by closing European ports. When Portugal resisted, he sent General Andoche Junot to occupy Lisbon. By 1808, Napoleon turned his attention to Spain, his former ally, forcing the abdication of the Spanish royal family and installing his brother, Joseph Bonaparte, as king. This move ignited a fierce, widespread insurrection across Spain, marking the beginning of a conflict that would last until 1814.

The turning point for the French came at the Battle of Bailén in July 1808, where General Pierre Dupont surrendered his entire army. This defeat shattered the myth of French invincibility and emboldened local resistance. The Spanish population adopted guerrilla warfare, launching constant raids and ambushes that crippled French supply lines. By 1812, although the French maintained over 350,000 soldiers in the region, nearly 200,000 were occupied solely with garrisoning territory and protecting lines of communication rather than fighting on the front lines.

The British, led by Arthur Wellesley—later the Duke of Wellington—capitalized on this French overextension. Despite early setbacks, such as the difficult retreat of Sir John Moore’s forces to Corunna in January 1809, the British maintained a defensive base in Portugal. Wellington utilized defensive works like the lines of Torres Vedras to frustrate French offensives. As Napoleon diverted resources to his 1812 campaign in Russia, the French position in the peninsula crumbled. Wellington’s victory at the Battle of Vitoria in June 1813 finally broke French control, allowing him to push into France and play a decisive role in Napoleon’s first overthrow.

General information, not professional advice. Spotted an error? Flag the card or see corrections and complaints.

Intellect Plus is a visual educational discovery platform for exploring fascinating facts and ideas. Brain Food, Not Brain Rot. What is Intellect Plus?

Related

• How a 'Spanish Ulcer' helped unravel Napoleon's empire
• How a decisive victory in Spain shattered Napoleon’s southern empire
• When Napoleon installed his brother as King of Spain, he ignited a national catastrophe.
• How a single defeat in 1808 shattered the myth of Napoleonic invincibility forever.
• How Napoleon's invasion of Russia turned into a catastrophic military disaster in 1812
• How a single tactical miscalculation shattered French dominance in the Peninsular War
• Why Napoleon called the Peninsular War his own personal Vietnam
• Why the 1813 Battle of Leipzig was the most significant clash of the Napoleonic era.
• How a lost tax collector helped create the world's first National Park
• How Napoleon dismantled the legendary Prussian army in just five weeks of warfare
• Evolutionary Deception: How Nature Weaponizes Senses to Survive
• Why Napoleon's momentum stalled at the bloody Battle of Eylau

More in History · Latest · A–Z · All topics