In 1807, Napoleon, the emperor of the French, along with his Spanish allies, invaded Portugal, and the Portuguese, largely out of concerns about their chances of successfully defending their nation, offered virtually no resistance. The royal family escaped when Napoleon's forces approached Lisbon in late 1807.

At the time, Brazil was Portugal's largest and most successful colony, and Rio was its administrative capital, and wealthiest city. Rio was the natural choice for the family to hide out in during the war.

The unintended consequence, though, was that by making Rio the capital of a global Empire, it gave the Brazilian people - primarily the aristocracy - a taste of autonomy. The Portuguese Royal Family remained in Brazil until a military uprising in Portugal in 1820-21 drew them back to Europe. When they left, however, it left things open for Brazilian "liberals" - large landowners and merchants to foment a movement for Brazilian independence. After a series of missteps by the Portuguese Cortes - their legislature - and the Portuguese military in Brazil, a member of that Royal Family, Dom Pedro, was able to ride a wave of nationalism to being coronated as Emperor of Brazil.

Source: Macaulay's Dom Pedro: The Struggle for Liberty in Brazil and Portugal, 1798-1834