The Russian Empire reached its height in this era. It is the largest country on the planet today and most of that land was acquired in this era. That being said, the AP doesn't require you to know too much about them, beyond that they are a land empire of the time and they sent peasants to the frontier to populate Siberia. So, know about the expansion and about Peter the Great, and that should be enough. Don't forget these guys, however. They are one of the most important states in the last, Contemporary Era (1900-present).