The Collapse of the Spanish Invasion – A Retreat Amidst Carnage (June 1762)

By June 1762, the Spanish invasion of Portugal had completely unraveled. Not only had the Spanish failed to achieve their main military objective—capturing Oporto—but their forces were now suffering catastrophic losses from disease, famine, and relentless guerrilla warfare.

A contemporary account describes the nightmare unfolding in Trás-os-Montes, stating that the stench of unburied Spanish corpses filled the mountains, as local peasants, motivated by hatred, refused to bury the dead. The scene bore striking similarities to the fate of Napoleonic soldiers decades later, who also met fierce Portuguese resistanceand suffered immense casualties.

Catastrophe in the Occupied Cities

Even inside Spanish-occupied cities, the invaders found no refuge. In Chaves, where 500 Spanish miqueletes had taken control on May 21, 1762, only 18 remained alive by the end of June—a stunning testament to the effectiveness of Portuguese irregular warfare.

Spanish Casualties and Desertions

According to the Spanish military historian José Luis Terrón Ponce, total Spanish losses during the first invasion of Portugal—from guerrilla attacks, disease, and desertion—exceeded 8,000 men.

Even more damning, General Charles-François Dumouriez, writing in 1766, estimated the figure at 10,000 casualties and explicitly recommended that Spain never again attempt an invasion through Trás-os-Montes, warning of the region's hostile terrain and population.

Final Collapse and Spanish Retreat (End of June 1762)

By the end of June, the Spanish position was untenable:

• The invasion’s primary objective—Oporto—remained out of reach.
• The Spanish army was decimated by starvation, disease, and guerrilla ambushes.
• Guerrilla forces had successfully cut off food supplies, worsening famine among Spanish troops.
• Portuguese regular forces, advancing from Lamego, threatened to cut the Spanish army in two—one force trying to cross to the southern bank of the Douro, the other still attempting to reach Oporto via the mountains.

Facing total collapse, the Spanish high command ordered a full retreat toward Spain, abandoning all their conquests except the city of Chaves, which they held on the frontier.

Conclusion: A Humiliating Defeat for Spain

The failure of the 1762 Spanish invasion of Portugal was one of the most disastrous campaigns in Spanish military history. Spain had underestimated Portuguese resistance, assuming that a swift invasion would force Lisbon to surrender. Instead, the Spanish lost thousands of men, saw their army harassed at every turn, and failed to achieve any meaningful strategic objectives.

By the end of June, the Spanish were not just retreating—they were fleeing for survival, marking the complete failure of their first attempt to bring Portugal into the Bourbon Family Compact by force.