Well, they conquered Spain by turning against them, allies up to that point, and having their army inside the country already. “The war can be said to have started when the French and Spanish armies invaded and occupied Portugal in 1807 by transiting through Spain, but it escalated in 1808 after Napoleonic France occupied Spain, which had been its ally.”

The the Spaniards handed Napoleon his ass on a platter, coining the word guerrilla.