How did the Spanish guerrilla movement resist French occupation?

During the Peninsular War, the Spanish guerrilla movement—a term derived from the Spanish word for "war"—played a critical role in resisting French occupation by engaging in unconventional warfare that complemented the efforts of British forces under the Duke of Wellington.12

Rather than engaging in large-scale conventional battles, which the Spanish regular army often lost, the guerrillas utilized tactics that severely hampered French operations. Their primary methods of resistance included:

• Pinning down garrisons: By maintaining a constant threat, they forced the French to commit significant manpower to guard duties and static defense.1
• Disrupting communications: They frequently intercepted French dispatches, preventing the coordination of enemy forces.1
• Isolating supply lines: They targeted and isolated French convoys, effectively wearing down the resources and morale of the occupying army.12

The movement was a complex phenomenon driven by diverse motivations; some participants were patriotic liberals, while others were motivated by a desire to defend traditional institutions and the church against French-inspired reforms. Ultimately, this persistent resistance, combined with Wellington’s strategic campaigns, forced Napoleon to divert vast amounts of men and money to the region, contributing to what he famously termed his "Spanish ulcer".12