How did Spanish and Portuguese guerrilleros assist the Duke of Wellington in the Peninsular War?

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Shiʿi militia fighter in Iraq — guerrilla warfare A masked Iraqi Shiʿi militia fighter dashing across a street, carrying a rocket-propelled grenade launcher, Baghdad, August 7, 2004.

During the Peninsular War, Spanish and Portuguese guerrillas played a vital role in supporting the Duke of Wellington by destabilizing French control over the region. While Wellington’s British troops engaged in major conventional battles, such as Vitoria, the guerrillas conducted an effective campaign of irregular warfare that significantly hampered French operations.1

The guerrillas assisted Wellington by performing several critical tactical functions:

Pinning down garrisons: By maintaining a constant threat, they forced the French to commit large numbers of troops to guard duties and static defense, preventing those forces from concentrating against Wellington’s army.1
Intercepting communications: They frequently captured French dispatches, which disrupted the enemy's command structure and provided the British with valuable intelligence.1
Isolating supply lines: By targeting and isolating convoys, the guerrillas severely strained French logistics, contributing to what Napoleon famously termed his "Spanish ulcer"—a drain on resources that ultimately undermined his broader European military efforts.1

Would you like to learn more about the specific tactics used by these guerrilla groups or the broader impact of the Peninsular War?