The whole region of Central and Eastern Europe has a complicated history. Empires have risen and fallen over many hundreds of years. Wars have been fought and borders have moved back and forth across the map.
Then there's also the pesky little problem of the nation state, which boils down to this: a state is a political entity, while a nation is a self-identified group of people. During the Treaty of Versailles negotiations in 1919 at the end of World War I, the idealistic US President Woodrow Wilson proposed the idea that every nation in existence should get its own state, unless the winners of the war decreed they weren't smart enough to manage themselves. It was more complicated than that, of course, and was glossed over in more polite terms, but in its essence, that was the basis of many terms of the treaty.
The Paris Peace Talks that led to the Treaty of Versailles met 145 times, during which the winners of the war debated where lines would be drawn that separated, for example, the state called Romania from one called Hungary, Austria from Czechoslovakia, and so on. They divided up Africa amongst themselves, and decided the Kurds didn't need their own homeland. Trouble was, the lines they drew didn't line up nicely with where "nations" lived, because people have a habit of moving around and mixing amongst themselves. As a result of how lines were drawn, many thousands of people had to pack up and leave the homes where they'd lived for generations because the "state" allotted to their "nation" was on the other side of a border from where they lived. Margaret Macmillan's terrific book, Paris 1919, describes the positions and negotiations in a way that clarifies many of today's political troubles.
While the idea of providing a homeland (or state) for every self-identified group of people (or nation) might sound fair or reasonable, nationalism itself tends to be thuggish. Its most extreme manifestations have been genocidal, as in Rwanda in 1994 or Germany from 1933-1945. State borders created by the Versailles treaty were meant to diminish conflict between nations. Some argue they did exactly the opposite.
In the wake of World War II and the Holocaust, the Marxist argument for ending the nation state entirely found fertile ground in some circles. At least in theory, the Soviets sought to build a single international union that would ultimately become one big happy family where differences of nation, language and culture fell away and all would be treated equally. As part of that plan they sent large numbers of Russians to settle in places from Ukraine to Lithuania to Turkmenistan. They also rounded up hundreds of thousands of nationals of those countries and shipped them off to Siberian work camps, where more than a million people died.
This isn't the only explanation for what's happening in Ukraine right now, but it's one point that should be better understood. Many Ukrainians - as well as nationals of other former republics - have bitter memories of when their parents or grandparents were sent to the camps. At the same time, Russians whose parents and grandparents were moved to one of the far-flung Soviet republics, were born and have lived their whole lives there. Other Russian-speakers in Eastern Ukraine have even deeper roots there.
As the Treaty of Versailles taught us, drawing lines between states in a way that will please nations is a difficult task at best, fraught with risks that may not be fully understood until it is far too late.