I've always assumed that among the Napoleonic wars, the Peninsular was singularly vicious. No doubt things like Goya's "Disasters of War" have compounded this assumption - but I've never seen art or literature from the other theaters that quite match the ferocity depicted. To be fair, Goya's a sort of singularly brutal artist, but most of the (admittedly wikipedia) reading I've done on the Peninsular war seems to confirm this. So: is it true that the Peninsular was was particularly violent? And, more importantly, what about Spain made it this way? Sure, guerrilla combat is always exceptionally messy, but why was Spain the place where this really seemed to take place?
Any book recommendations on the peninsular war would also be greatly appreciated.
Your best resources for the Peninsular war, to get started at least, would be The Spanish Ulcer by Gates and A History of the Peninsular War by Oman.
To answer your question, yes it was very brutal. The brutality begins with Murat's suppression of the Madrid revolt(which resulted from Napoleon's decision as mediator to make his brother Joseph King of Spain instead of Ferdinand or Charles). As the Imperial Guard was stationed there, including Napoleon's famed Mamelukes, it was a powder keg waiting to go off when you consider these facts: a)Spain was fervently Catholic and the Inquisition and the Clergy were a powerful force in Spain, b)Catholic Spain had for centuries been in conflict with Muslim forces from Africa. Therefore when the Spaniards saw these Mamelukes killing Spaniards (who were in open armed revolt against the French soldiers) it incited a national rebellion.
The Spanish were mercilessly brutal throughout the campaign. There are recorded instances of French soldiers being buried up to their necks only to have their heads covered with fire ants. Atrocities and counter-atrocities would be committed for the course of the entire campaign.
Part of this feverish resistance was blood lust incited in the peasantry by their Feudal and Catholic masters. These institutions were what Napoleon hoped to wipe out(he protected the Church's existence but would greatly diminish its power), as he had done everywhere he spread the principles of the revolution up to this time. Therefore when Napoleon seized the Spanish throne, the remaining Spanish powers, the Church and the Nobility, taught these mostly illiterate peasants that the French and Napoleon were the physical embodiment of Satan.
Sources used: The Spanish Ulcer: A History of the Peninsular War, David Gates
Blundering to Glory, Owen Connelly.