Russia had an empire until 1992. Now it doesn't. This is over simple but it is very true.

The Russian people originated around Kiev, which is in modern day Ukraine. They had a nice big kingdom and due to the way their inheritance system worked, this kingdom fragmented. Then the mongols came, and Russians were subjugated to them. Novgorod and Moscovy managed to resist by being too far away for regular smashing. Then when Moscovy managed to establish a Russian empire they retook the Ukraine and colonized it. Over time the people in Ukraine drifted from the other Russians, different climate, distance, and other factors created a different culture. When the USSR was formed, Ukraine was set up as a Republic within the Union. in 1992 it became it's own place, the Ukrainians had identified themselves as different from Russians. This is like how Americans are different from English, even though they share a common root and even a history before a specific date. But not everyone living in the land of The Ukraine see themselves as Ukrainian, some think of themselves as Russian. And with nationalism in play, "Russians should be in Russia" and there you go.

I am leaving out other issues because they are things I don't know. But this is the basic historical overview. I left out lots of details.