Enormous volumes of documents were destroyed during the last days of Nazi Germany. What information was lost permanently to history?

Even looking at just my own little narrow focus of Napoleonic era armies, there were immense losses due to the war.

Prussia

The archives of the Prussian war ministry prior to 1919 were almost entirely destroyed as a result of a fire caused by an Allied bombing raid on the city of Potsdam near Berlin. Very little survived – 31 shelf metres of documents relating to the early development of military air power which had been transferred to the Air Force archives and very few personnel files, mostly of historical significant individuals like Scharnorst, Gneisenau, Roon and Moltke the Elder. What makes this even more tragic is that the raid occurred on April 14th, 1945 – just 3 weeks before the end of the war with the Soviets within artillery range of Berlin. This was literally the last major mission RAF Bomber Command conducted against a German city.

This means that the history of the Prussian of army during the Napoleonic wars (and all of its wars) is largely dependent on the same sources that have been used for the past two centuries – memoires and regimental histories mostly written in the mid-to-late 19th century that need to be treated carefully given the nationalist strains that permeate them. Anecdotally, modern histories of the Prussian army compare poorly to what is being written about the British, French or Russian armies. It would be impossible to conduct any kind of wide ranging demographic studies – for example, Martinien’s exhaustive compilation of French officers killed or wounded during the wars could never be replicated, nor could Houdaille’s statistical study of the fates of enlisted men from the French army archives; if you have an ancestor who served in the French army in the 18th century, you can download their service record from the French Army Archives (known under the delightful acronym of SHAT) but there are only a few thousand records still extant for the entire history of Prussia. This limits the possibilities for modern writing on the Prussians - the type of modern social history written about other armies would be extremely difficult for example, or a retelling of the Prussian army during the Waterloo campaign without relying on the same old overused sources (witness the tired retreading of 19th century nationalist claims by Peter Hofschroer in the early 2000s that ended up with him being successfully sued for defamation by a British historian - access to the original archives would have cut that nasty debate off very quickly).

This isn’t even the greatest loss, that would be the loss of all the records of the German General Staff in the period leading up to World War I, which scholars had barely any time to examine before the destruction of the archives

Other German States

The fragmented nature of pre-unification Germany meant that historical records were located in cities all over the country. Aerial bombardment, both the fires themselves and accidents during the transport of documents to safer locations, led to the destruction of innumerable records from the independent states that had existed prior to Germany – Bavaria lost 20,000 dossiers of documents relating to army, the records of Hesse-Darmstadt and Wurzburg were almost destroyed, as were 19th century Hanoverian records and those of the Electorate of Mainz.

Naples

As u/Alkibiades415 outlined, the Germans made a deliberate effort to destroy the libraries and archives of the Italian city of Naples. This had been an independent Kingdom prior to Italian unification and during the Napoleonic period a French client kingdom had been set up, initially ruled by Napoleon’s brother Joseph and subsequently his brother-in-law Joachim Murat. The German destruction of records included much of the records of this Kingdom, to the extent that the official history of the army of Kingdom sponsored by the Historical Section of the Italian Ministry of Defence includes an entire section of the Bibliography dedicated to sections of the archives previously seen that had been destroyed during the war. This includes all the regimental records, the records of the military academies, the navy, records of British landings, correspondence records, military memoires of campaigns written immediately after they happened and records of Neapolitan formations who were sent to the campaign in Spain, the Tirol, Russia, Poland and Germany.

Taking one example of what was lost, the 7th Line Regiment of the Neapolitan army, the Real Africano regiment, was built from a unit transferred from the French army – the pionniers noirs. This was a unit originally formed from prisoners taken during the Haitian uprising and joined the campaign in Northern Germany in late 1813, where the locals were amazed to see unit of made up of People of Colour, including their officers – practically unheard of in 19th century Europe. Unfortunately there is practically no records of these men in their own voices and we’re largely reliant on 19th century narratives from European witnesses and all the racist baggage that entails.

Kingdom of Italy

An allied bombing raid on Milan in 1943 damaged material from the Archivio di Stato that was in the Palazzo del Stato while it was being transferred to a safer location. Part of the records of the Napoleonic Kingdom of Italy was included in the destroyed material, resulting in the loss of large amounts of material related to Royal Guard of the Kingdom

Duchy of Warsaw

The records of the Duchy held in Warsaw were considered to have been almost totally destroyed by the Germans as a result of the deliberate destruction of Polish heritage following the 1944 Warsaw Uprising.