As a student of American history, I didn't have to learn much about changes in territorial sovereignty. There was the Louisiana purchase from France, the war with Mexico, the dispute over our northern border with the British, the purchase of Alaska, and the annexation of Hawaii. Maybe there had been six flags over Texas, but it didn't mean much to the Texans I met.
Then I began to learn about Europe, where centuries of conquests and re-conquests have redrawn borders, often with killings and expulsions of the losers. In spite of my impressive academic credentials, I only recently began to understand, for example, the enormous disruptions that accompanied the fall of the Ottoman empire. For a longer time, I had been aware of some of the changes that occurred in Central and Eastern Europe. But they are still astonishing to contemplate.
One of the best illustrations of what happened is the story of Lviv, formerly Lvov and formerly Lemberg. On a visit there a few years ago, I noted that my students could encounter a person my mother's age [she was then in her mid-80s] who had been born in Austria-Hungary, went to school in Poland, got married in Germany, retired in the Soviet Union and now lives in Ukraine -- all while living in the same house on the same street.
Similar tales could be told of other towns, but Lviv reinforced for me the important lesson that people may harbor historic memories and loyalties that may run counter to their current sovereigns.