Frances decision to occupy Spain saw its troops were stuck in a war that it couldnt get out of. From 1808 to 1814, French forces battled British, Spanish, and Portuguese regulars up and down the Iberian Peninsula, along with Spanish guerrillas who continued their resistance to French occupation. The war starved Napoleon of the troops he needed to fight his enemies across Europe.The Continental System and the Portuguese TrapAn illustration of smugglers violating the Continental System by John Augustus Atkinson, 1808. Source: Wikimedia CommonsIn 1806, Emperor Napoleon I of France was on his way to becoming the master of continental Europe. However, he faced a major challenge at sea, as the British Royal Navy blockaded French ports and occupied French overseas colonies at will.In response, Napoleon instituted the Continental System, a scheme designed to prevent British ships from trading with continental Europe, while developing industry in the European continent. Starting with the Berlin Decree of 1806, the emperor prohibited his subjects and allies from trading with Britain. This was followed by the Milan Decree of 1807, which stated that any ship that consented to a British inspection or sailed into a British port could be seized by French authorities.The Continental System proved to be a major failure. Merchants all across Europe continued to trade with the British. Many French officials proved to be unwilling to address the smuggling, including Napoleons brother Louis in the Netherlands. British naval power ensured that new trade routes could be opened to the Americas and Asia. Most disastrously, Napoleons efforts to enforce the system led him to start several new wars despite defeating his major continental rivals by 1807.Portugal was Englands oldest ally in mainland Europe and it refused to stop trading with the British. As a result, Napoleon issued an ultimatum to Portugals Prince Regent, the future King Joo VI. The Portuguese intentionally dithered, enraging France and causing Napoleon to authorize an invasion of the territory. Spain, at this time an ally of France, consented to allowing French forces to cross its territory. While General Jean-Andoche Junots 30,000 strong Army of Portugal suffered losses from attrition as they advanced on Lisbon, they seized the capital and while the Portuguese royal family evacuated to Brazil aboard British ships.The Abdications of Bayonne: A Political BlunderA portrait of Charles IV of Spain before his abdication by Francisco Goya. Source: Museo del Prado, MadridSpains decision to allow French forces to cross its territory proved to be a blunder. By early 1808, some 100,000 French soldiers poured into Spain and began establishing themselves near major Spanish cities. Napoleon mistrusted the Spanish and he hoped to exert as much control over Spanish policy as possible. To many Spaniards, it became clear that France was taking advantage of Spains friendship, fuelling resentment towards the French garrisons and the pro-French chief minister Manuel Godoy.Napoleon confirmed the worst fears that many Spaniards had when he began meddling in the affairs of the Spanish monarchy. In March 1808, supporters of the heir apparent, Ferdinand VII, forced his unpopular father, King Charles IV, to abdicate. Charles later changed his mind and sought to reclaim his throne. Both Ferdinand and Charles appealed to Napoleon for support. Napoleons brother-in-law Joachim Murat, commander of the army in Spain, encouraged both men to travel to Bayonne over the border in France, where Napoleon would arbitrate between them.However, the emperor deceived both of them, forcing Ferdinand to recognize his father as king and then forcing Charles to abdicate. Upon doing this, Napoleon declared the Spanish throne vacant and recognized his brother Joseph as the new king in July. Both Bourbons stayed in captivity until 1813.Even before the two kings were forced to abdicate, Spanish resentment against French interference resulted in the outbreak of riots in Madrid on May 2, 1808. While Murat brutally crushed the Spanish rioters, this revolt became the catalyst for the Peninsular War.Why Bailn and the Spanish Ulcer Changed EuropeThe Surrender of Bailn by Jos Casado del Alisal, 1864. Source: Museo del Prado, MadridAfter the imposition of King Joseph, large numbers of Spanish troops refused to swear allegiance to the new French regime. As several bodies of French troops moved to subdue the remaining Spaniards, in July 1808 General Pierre Dupont led his II Corps of 20,000 men into Andalusia, where the regional governments in Seville and Cadiz had refused to submit to King Joseph.Dupont was confident that the Spanish forces against him were an ill-trained motley crew that were no match for the professionalism of the French army. However, a combination of difficult terrain and Spanish resistance proved fatal to his army. When he decided to turn back in the face of Spanish hit-and-run attacks, he found a force numbering around 30,000 men under the command of General Francisco Javier Castaos.The Spaniards proved to be a formidable adversary and they were assisted by the desertion of several Swiss regiments in the French army. When Dupont ordered assaults on the Spanish lines, his men took heavy losses and were gradually encircled. By July 19, he agreed to surrender with his remaining 17,000 men. Instead of being repatriated to France, they spent the rest of the war in Spanish captivity.The destruction of Duponts corps proved to be cataclysmic for Napoleons ambitions. Many members of the Spanish elite who had wavered in their loyalty decided to join the juntas in Seville and Cadiz. Joseph panicked and fled towards the Ebro River, before regaining his composure and returning to Madrid. Britain began landing troops in Portugal to expel Junot from Lisbon. Elsewhere in Europe, Austria began rearming and anti-Bonapartist movements in Germany prepared to fight as well. In late 1808, Napoleon personally assumed command of the French troops in Spain to reimpose French control.The Birth of Guerrilla: Why the French Could Not WinJuan Martn Dez, one of the most famous Spanish guerrillas who fought the French occupiers, 1811. Source: The History AvenueDespite their success at Bailn, most Spanish regulars struggled in battle against the French. However, many ordinary Spaniards vowed to make their land inhospitable to the occupation forces. This led to the rise of the Spanish guerrillas and one of the worlds first organized insurgencies. From 1808 to 1814, tens of thousands of Spaniards fell on French positions far beyond the actual front lines.The guerrillas were a mix of professional soldiers, local peasants, and urban dwellers from across the political spectrum. They were united in their desire to reassert their sovereignty over their country. Initially, they were armed with a heterogeneous mixture of weapons, but over time they were more standardized. Using the rugged terrain to their advantage, the guerrillas struck French supply convoys, outposts, and columns sent to hunt them down. While the French did have counterinsurgency experience from crushing the Vende uprising in western France, they were never able to subdue the guerrillas in Spain.At the height of the war, France had between 250,000 and 300,000 men stationed in Spain. However, only 70,000 of these men were available on the field. The remainder were assigned to garrison duty to fend off the guerrillas, tying down valuable manpower that Napoleon could have used elsewhere. The Spanish guerrillas proved to be a perfect case study of how a sustained insurgency could wear down a professional military force over time.The British Intervention: Wellingtons Long GameAn illustration of Wellingtons victory at the Battle of Talavera in 1809 by William Heath, 1815. Source: Wikimedia CommonsIn conjunction with the Spanish guerrillas and regular army, the British government sent Arthur Wellesley, later Duke of Wellington, to command an Anglo-Portuguese force to drive the French out of Portugal. In August 1808, Wellesley defeated Junot at the Battle of Vimeiro to expel the French from Portugal. In 1809, he joined a Spanish force to defeat the French at the Battle of Talavera, where he was ennobled as Viscount Wellington.When Marshal Andr Massna led an army back into Portugal in 1810, Wellington retreated behind a set of fortifications known as the Lines of Torres Vedras. Massna was forced to retreat to Spain, and a further attempt in 1811 proved equally fruitless while Wellington continued to build up his forces for a counteroffensive.By 1812, with Napoleon preoccupied with his campaign in Russia on the other end of Europe, Wellington launched an offensive into Spain with the intention of pushing towards Madrid, where King Joseph was reliant on the advice of Marshal Jourdan and struggled to control the other marshals, who acted as independent warlords. Wellington made good progress, decisively defeating a French force under Marshal Auguste Marmont at Salamanca in 1812 and briefly taking the capital. However, he decided to retreat to Portugal after unsuccessfully besieging the city of Burgos.When Wellington crossed into Spain for the last time, he marched into the northern mountains where Joseph Bonaparte had not expected him to go. Fearing that he would cut off Frances supply lines by seizing the foothills of the Pyrenees, Joseph and Jourdan marched out from Madrid to meet him. The subsequent Battle of Vitoria, fought on June 21, 1813, proved to be the death knell of the French occupation of the Iberian Peninsula. The surviving French forces engaged in a fighting retreat across the Pyrenees while Wellington advanced into southwestern France. In December 1813, Napoleon agreed to acknowledge Ferdinand VII as King of Spain.Did Spain Cause Napoleons Downfall?The Battle of the Pyrenees by William Heath, 1814-15. Source: City Library of ToulouseWhen people consider Napoleons most disastrous error, the decision to invade Russia in 1812 comes to mind. It isnt hard to see why; he lost hundreds of thousands of men and he lost some of his credibility as a leader. However, his intervention in the Iberian Peninsula proved to have an even worse long-term impact. By manipulating the Spanish elite, he turned most of the country against him and lost one of Frances closest allies. Additionally, he failed in his initial objective of stopping the smuggling of goods from Spanish and Portuguese ports to Britain.Every leader who desires to control a large empire needs to have the resources and manpower available to do so. Napoleon learned this lesson the hard way when he sought to fight a series of conflicts with his rivals in Central and Eastern Europe while simultaneously occupying parts of Northern and Southern Europe. Having hundreds of thousands of men tied down fighting in the Iberian Peninsula did not bring the French any strategic gains. It did, however, deprive France of much-needed manpower for the invasion of Russia.The imposition of the Continental System was an attempt to even the odds between France and Britain. However, British naval superiority ensured that Napoleons plan would fail. Spain may not have been Napoleons most effective ally, but it was friendly with the French nonetheless. Napoleons actions also breathed new life into Spanish nationalism when many Spaniards were asking themselves what the future of their country would be. It is fair to say that invasion of Spain was one of the worst decisions made by Napoleon during his career.