Why did Britain send Arthur Wellesley to Portugal in 1808?

Britain sent Arthur Wellesley to Portugal in 1808 to challenge the French occupation of the country, which had been seized by General Andoche Junot in 1807 as part of Napoleon’s efforts to enforce the Continental System against British trade.12

Following the Peninsular War outbreak in Spain, the Iberian Peninsula became a critical theater where Britain could actively oppose French hegemony on land. By landing 13,500 troops in Mondego Bay in August 1808, Wellesley provided essential military support to the Portuguese and Spanish resistance. His subsequent victories at Roliça and Vimeiro forced the French to evacuate Portugal under the Convention of Sintra, effectively establishing a vital British bridgehead on the continent that served as a base for future campaigns against Napoleon.1324