Final answer:
In 1808, to coerce Portugal into adopting the Continental System, Napoleon invaded Portugal through Spain, sparking the Peninsular War.
Step-by-step explanation:
In 1808, Napoleon invaded Portugal to enforce the Continental System, mobilizing troops through Spain and eventually triggering the Peninsular War.
To get Portugal to accept the Continental System, Napoleon needed to invade it. This required traversing Spain, which was agreed upon due to its status as a French satellite. The invasion had profound repercussions, resulting in Spanish insurrections, the eventual involvement of British forces, and a protracted conflict known as the Peninsular War, characterized by guerilla warfare and rising Spanish nationalism.
This move by Napoleon not only targeted Portugal's non-compliance but also ultimately disrupted his alliance with Spain, turning the Spanish monarchy against him when he placed his brother Joseph on the throne.