The bulk of Russia's eastern territories was aquired during the 17th and early 18th century. This kind of blows my mind. We're talking about roughly half a continent, a territory more than four times larger than the Roman Empire during it's peak.
How did they manage to control and hold onto such vast and hostile land in a time before the railway and the telegraph? And equally as important: What was their motivation to expand so far east? What did they hope/expect to gain?
This answer I wrote recently touches on this question a bit. The long and short is that the expansion was first and foremost to secure furs as tribute from native peoples for international sale.