I have read many not completely satisfactory "explanations," most of which are repeated in other comments. Those may be part of the story. The most convincing s

I have read many not completely satisfactory "explanations," most of which are repeated in other comments. Those may be part of the story. The most convincing story that I have seen is as follows: Ukraine is very similar culturally to Russia. Putin cannot afford to have a successful democratic Ukraine on his borders as an example to Russian citizens. They might decide that Putin's way of governing is not actually the best for the Russian people. So, he does everything he can do to undermine a democratic Ukraine. If a western style democracy fails in the Ukraine, well, clearly it is not suitable for Russia either. Ideally, he would like Ukraine within Russia's sphere of influence as Belarus is, but I am not sure that is vital. This is actually fairly consistent with how the Soviet Union behaved in Eastern Europe. Romania and Yugoslavia moved away from the Soviet Union, but they were dictatorships, so they were left alone. Anytime there was even a glimmer of democracy, however, the tanks were sent in.