Mostly through people still living in the same place remembering the glory days of old. If you look at Poland and Lithuania, which became Poland-Lithuania, whi

Mostly through people still living in the same place remembering the glory days of old. If you look at Poland and Lithuania, which became Poland-Lithuania, which roughly split so that Poland went into Prussia and Lithuania into Russia; where Prussia lasted for 100 years and ended with WW1 where Poland re-emerged and had their common sense of identity enhanced by Hitler almost immediately re-invading, and Lithuania existing as part of the USSR for some additional 100 years before breaking out and doing their own thing. There's still a cultural identity in these places. The people living in them weren't replaced or relocated, primarily the flag and regent.