Masséna had to order a retreat at the beginning of March 1811 having lost 21,000 men from hunger and disease induced by the scorched earth policy of Wellington.

[1][2] On 11 October 1810, Massena with 61,000 men found Wellington behind an almost impenetrable defensive position, the Lines of Torres Vedras consisting of forts and other military defences built in absolute secrecy to defend the only path to Lisbon from the north.

The following day, Junot's troops seized an outpost belonging to Brent Spencer's 1st Infantry Division, but were quickly ejected from the position by a British counterattack.

But the lack of food and fodder meant that Masséna was forced to retreat northwards, starting on the night of 14/15 November 1810, to find an area that had not been subjected to the scorched earth policy.

The French held out through February although the Iberian peninsula had suffered one of the coldest winters it had ever known, but when starvation and diseases really set in, Masséna ordered a retreat at the beginning of March 1811 having lost another 21,000 men.