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Why did Napoleon I begin the Peninsular War in 1808?

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Jacques-Louis David: The Emperor Napoleon in His Study at the Tuileries — Napoleon I *The Emperor Napoleon in His Study at the Tuileries*, oil on canvas by Jacques-Louis David, 1812; in the National Gallery of Art, Washington, D.C.

Napoleon I initiated the events leading to the Peninsular War primarily to enforce the Continental System, an economic strategy designed to cripple Britain by cutting off its trade.1

After his pact with Russia in 1807, Napoleon sought to isolate Britain by forcing neutral or friendly powers, specifically Portugal, to close their ports to British ships and declare war on the nation. When Portugal proved hesitant, Napoleon sent troops through Spain to occupy the country.1

As French forces moved through the Iberian Peninsula, they began occupying strategic Spanish fortresses, revealing Napoleon’s broader ambition to exert direct control over the region. Taking advantage of internal political instability—specifically the Revolt of Aranjuez and the subsequent power struggle between Charles IV and Ferdinand VII—Napoleon forced both monarchs to abdicate at Bayonne in May 1808. He then installed his brother, Joseph Bonaparte, on the Spanish throne, triggering a widespread popular uprising that marked the beginning of the war.12