I am a master student of history and studied on an aspect of how mythology was used in the 19th century, so I will answer this from a Dutch perspective. I specifically looked at how the writings of Tacitus and Caesar were used to represent national ideas, where I compared Dutch and Belgian historiography.

On a whole historians were looking to find some kind of 'national essence', as was forumulated in the 19th century and recognize those traits in the ancient historiography. For instance: The Dutch considered themselves to be a seafaring country that found it's origins in the rebellion against Spain in the 16th century. Therefore they looked at the peoples that Tacitus placed in the general area of the modern Netherlands: The Caänites, the Batavians and the Frisians.

The Caänites were said to be a skillfull seafaring people that got most of their sustenance from the sea so that gets mentioned. Mainly, though they looked at the Batavians as they were 1. A powerfull ally of the Romans, that got to supply bodyguard for emperors 2. They revolted against the Romans when their priviliges got cut. This Batavian revolt, under Julius Civilis, was represented as an innate sence of individuality versus the power of Rome (read: Catholic Spain).

The subject of my study was the representation of Tacitus in Dutch and Belgian historiography before and after the Belgian independance. You can clearly see (though mostly in more 'popular' histories) that at first the Belgian tribes were represented as being included in the revolt (which they were), but that after the Belgian independance more focus was put on the fact that some of the Belgian tribes left the movement when the Romans sent up additional legions. Basically saying: 'even back then the Belgians were already traitors'.

The Belgians, by the way didn't look at the Roman history that much but saw the origin of their people more in the Catholic era (thereby distancing themselves from the Dutch protestants).

Keep in mind that there isn't a shred of evidence that those tribes kept living in the area that Tacitus puts them in after the great diaspora of 'barbarian' people during the late roman era and the early middle ages. So any kind of precursorship is only projection.

So, in short ancient sources were used to underline what was perceived as national traits in the 19th century, and there was some cherrypicking and embellishment going on. A good book on how this historic essentialism got overinterpreted to a fault is A most dangerous book : Tacitus’s Germania from the Roman Empire to the Third Reich , which focus lies on German representation.

Sources:

• Krebs, Christopher, A most dangerous book : Tacitus’s Germania from the Roman Empire to the Third Reich, 2011.

• N.C.F. van Sas, 'De mythe Nederland', De negentiende eeuw 16 (1992)

• Swinkels, L, De Bataven: verhalen van een verdwenen volk (Amsterdam, 2004).

• Mathijsen, Marita, 'De paradox van het internationale nationalisme in Nederland 1830-1840', (Hilversum, 2010).

• Schöffer, I, 'The Batavian myth during the sixteenth and seventeenth centuries' (1975).