Hetalia Headcanons

Germany

Under a certain set of conditions, personifications can actually die separately from the physical country.

This only happens when a personification has no form of energy left, physical (like from food) and 'national' (from their peoples' patriotism, haven't been to their land in a while, or etc).

The personification gets reborn as a young child and ages normally until they reach the age they would be if they'd never died, but do not have their memories. The only way to get their memories back is to meet a personification they had been close with during their previous life.

The best example of this is, as you can guess, is Germany. He was fatally wounded during the Battle of Austerlitz, unable to recover due to poor nutrition and low troop morale.

The personification that would have given him his memories back was Germania. Unfortunately, Germania had faded by that point. Germany did later meet with Germania, but since Germania was a ghost by that time and not a personification, there was no effect. That's why he can't remember his days as HRE.