With the occupation by Napoleon, Portugal began a slow but inexorable decline that lasted until the 20th century. This decline was hastened by the independence in 1822 of the country's largest colonial possession, Brazil. In 1807, as Napoleon's army closed in on Lisbon, the Prince Regent João VI of Portugal transferred his court to Brazil and established Rio de Janeiro as the capital of the Portuguese Empire. In 1815, Brazil was declared a Kingdom and the Kingdom of Portugal was united with it, forming a pluricontinental State, the United Kingdom of Portugal, Brazil and the Algarves.

Under whose occupation did Portugal begin a slow decline?