Birthed in blood and hounded by war from its very beginning, the French Republic died its first death in 1804 with the ascension of Le Petit Corporal, Napoleon Bonaparte, to the throne as the leader of the First French Empire. As his rise to emperor was strongly related to his military expertise, it’s no surprise that war would continue for France. Bolstered both by his military genius and, often, the incompetence of many (though certainly not all) of his foe nations, Napoleon’s army would steadily advance across Europe.
By 1807, the French Empire controlled the large swaths of Europe both directly or indirectly after victory in the War of the Fourth Coalition against a large number of allied nations. Unfortunately for the would-be ruler of Europe, he lost any effective control of the seas after a large-scale defeat at the hands of English Admiral Horatio Nelson’s fleet at the Battle of Trafalgar in 1805. In addition to halting any attempt at direct French invasion of England, British naval dominance doomed the Continental System, a French attempt to cut off the United Kingdom and their European allies from trade to failure.
Portuguese resistance to the Continental System provided an excuse for the French, with the planned aid of their then Spanish allies (many of the ships lost during the Battle of Trafalgar were Spanish), to take control of Portugal. The French, however, already distrustful of the Spanish government and with their own plans to control Spain more directly, marched troops into the country. Though these troops were sent to allegedly aid in dealing with the Portuguese, the French soon started taking control of Spanish cities. The divided and dispirited government was further paralyzed by French-backed coups and other attempts to undermine the Spanish government. On May 5, 1808, Napoleon was in a position to directly install his brother, Joseph, as King of Spain.
A few days earlier, on May 2nd, Spanish citizens in Madrid rebelled against the occupying French. The brutal French response, both in Madrid, and in other cities, combined with discontent over the installation of King Joseph, ended any chance the French had of gaining smooth control of Spain. Resistance spread throughout Spain. By August, the British, inspired in particular by Spanish resistance, sent military aid to Spain and Portugal. This Peninsular War (named after the Iberian Peninsula containing Spain and Portugal) would rage until 1814.
Conventional warfare was a large component of the Peninsular War, but perhaps most striking in the context of the greater series of Napoleonic Wars, was it being marked by what is popular to now define as asymmetric warfare, or what is more commonly known as guerrilla warfare (note the word itself means “little war”). Although the guerrilla actions of the Peninsular War are certainly not the first example of guerrilla warfare in history, the sheer scale of the activity and its success against the preeminent conventional nation-state military power of the day provided an example for others in following centuries, and, of course, the Spanish word came into English to describe the style of warfare.
These guerrillas eschewed field battles. Instead they concentrated on hit-and-run and surprise attacks. They could kill the enemy or raid for supplies then fall back into mountainous terrain where the French regulars would not only have difficulty following but also difficulty fighting them with conventional tactics.
Although the French committed more than a few atrocities during their time in Portugal and Spain, it should be noted that the Spanish guerrillas didn’t confine their efforts to military targets. People deemed too pro-French would often pay with their lives. As one might expect, the guerrilla forces were not particularly organized and did not always operate under tight central authority or control. In addition, as is also common throughout the history of warfare, many men of ill repute viewed guerrilla warfare as an opportunity to satisfy their criminal or violent urges under the false flag of patriotism. If a group of guerrillas came to a village and “liberated” excess supplies or hurt someone, it wasn’t as if innocent civilians could just file a grievance with the government. With the French taking their own brutal invaders’ liberties with the countryside and also often doubling-down on their efforts due to irregular activity, it was not a pleasant time to be a civilian.
Though guerrilla warfare was a major contribution to the eventual victory over the French in the Peninsular War, it was not, in of itself a sufficient factor. Definitive conventional victories were still required. That being said, the guerrillas made life hell for the French regular forces. Reprisal and control attempts only further enflamed the populace. The irregular forces ambushed forces, disrupted supply lines, assassinated officers, seized messengers and otherwise thoroughly undermined the French forces. Beside the direct causalities inflicted, guerrilla activities prevented large chunks of French forces from being employed at full strength in either Spain or Portugal. Instead, many French troops had to be distributed to try and respond to these constant irregular attacks. Thus, the guerrilla efforts acted as an effective force multiplier for the anti-French Allies and also played a key role in delaying the French invasion timetable. Among other things, this also allowed the British time to bring sufficient resources into the war effort.
While Napoleon had made statements that the French would be able to conquer Spain and Portugal with only a minor expenditure in troops and effort, as the Peninsular War dragged out, the French emperor recognized the war instead as his “Spanish ulcer”.