Britain's involvement in the Peninsular War was the start of a prolonged campaign in Europe to increase British military power on land and liberate the Iberian peninsula from the French. In August 1808, 15,000 British troops—including the King's German Legion—landed in Portugal under the command of Lieutenant-General Sir Arthur Wellesley, who drove back Henri François Delaborde's 4,000-strong detachment at Roliça on 17 August and smashed Junot's main force of 14,000 men at Vimeiro. Wellesley was replaced at first by Sir Harry Burrard and then Sir Hew Dalrymple. Dalrymple granted Junot an unmolested evacuation from Portugal by the Royal Navy in the controversial Convention of Cintra in August. In early October 1808, following the scandal in Britain over the Convention of Sintra and the recall of the generals Dalrymple, Burrard, and Wellesley, Sir John Moore took command of the 30,000-man British force in Portugal. In addition, Sir David Baird, in command of an expedition of reinforcements out of Falmouth consisting of 150 transports carrying between 12,000 and 13,000 men, convoyed by HMS Louie, HMS Amelia and HMS Champion, entered Corunna Harbour on 13 October. Logistical and administrative problems prevented any immediate British offensive.

Meanwhile, the British had made a substantial contribution to the Spanish cause by helping to evacuate some 9,000 men of La Romana's Division of the North from Denmark. In August 1808, the British Baltic fleet helped transport the Spanish division, except three regiments that failed to escape, back to Spain by way of Gothenburg in Sweden. The division arrived in Santander in October 1808.