Major-General Sir William Erskine, 2nd Baronet (30 March 1770 – 15 May 1813) was an officer in the British Army, served as a member of Parliament, and achieved important commands in the Napoleonic Wars under the Duke of Wellington, but ended his service in insanity and suicide.

The administrators of the army at Horse Guards responded that, "No doubt he is sometimes a little mad, but in his lucid intervals he is an uncommonly clever fellow; and I trust he will have no fit during the campaign, though he looked a little wild as he embarked.

At Casal Novo on 14 March 1811, Erskine advanced his men along the main road in fog without proper scouts.

[6] At the Battle of Sabugal, the fog and Erskine's bungling saved General Jean Reynier's isolated French corps from destruction.

The hapless Erskine, who was very nearsighted, issued a set of foolish orders then promptly got lost in the fog with the cavalry, allowing the French to escape from Wellington's trap.

[9] From 19 June 1811, Erskine led four mounted regiments in the newly organized 2nd Cavalry Division in Rowland Hill's corps.