Viscount Wellington arranged for the defence of Portugal by massing the Anglo-Portuguese army at two points—the first, under his own command, on the Mondego, and the second, under General Hill, on the Tagus. He also issued orders for the construction of the Lines of Torres Vedras. The Light Division under General Craufurd was pushed forward to the Agueda as a Corps of Observation.

Massena, with a mobile force of 65,000 men, now threatened Portugal, having captured the fortress of Ciudad Rodrigo, defended by the Spaniards.

The first fight of the campaign was on 10th March, when General FÉrey endeavoured to surprise the Light Division by a night attack on the bridge of Barba del Puerco; this was held by the 95th Rifles, who drove back the French with heavy loss.

Craufurd maintained his position on the Agueda for four months after this, but in July, Ney with 30,000 troops advanced on Almeida, and Craufurd elected to stand and fight him with the Coa River behind him, and being vigorously attacked by overwhelming forces, withdrew with great difficulty and with heavy loss. The fortress of Almeida shortly afterwards fell into the hands of the French, and Massena advanced on the line of the Mondego with the object of driving the British army out of the country. Wellington, however, barred his advance at Busaco, and a sanguinary battle was fought, in which the French were worsted. Massena, finding he could not force the position at Busaco, manoeuvred to his right and threatened Wellington's line of retreat, and the latter fell back to his prepared defences of Torres Vedras.