Marshal Masséna's defeat in his invasion of Portugal in 1810 was quite catastrophic, it was a bit of a miniature version of the Russian campaign. Started of well for the French but the scorched earth tactics of the Anglo-Portuguese followed by the retreat behind the impregnable Lines of Torres Vedras scuppered any hope for Massena. He withdrew after a month, though probably should've bailed earlier, after finding no weakness at all in the fortifications. The only redeemable aspect of the retreat was Marshal Ney's great rearguard action. After entering Portugal with around 65,000 men, 25,000 were lost, the vast majority due to hunger and disease, it tarnished the reputation of Massena, who was one of France's greatest commanders.