"late on in the war"

Hi,

― This was quite late on in the war and  there were several camps for prisoners of war in Wale….

I'd like to ask you what this "on" is.

I think this is an adverb, not a preposition, but anyway I don't understand what it means.

'Late on' is a pretty standard although quite informal expression. The 'on' really just intensifies/stresses the idea of 'late'. 'Late on in the war;' sounds like it is close to the end of the war.

You can also use the rather opposite expression, 'early on', meaning near the start.

Best wishes, Clive

You can also use "later on" and "earlier on" to place the time of action, at any point in the range of possibility. [He enlisted in the army because earlier on in the war his friend had done the same.]