If you're wondering why Putin's building up forces on the Ukrainian border, here's an article by John J. Mearsheimer that lays the whole thing out. He's a giant in the international relations world, to the point where the notoriously hawkish and pro-western Foreign Affairs actually prints his articles that are critical of NATO simply because of the prestige he has; he's the leading member of the realist school of thought, which is a refreshing glass of cold water in a world dominated by liberal hegemony.

If you're looking for a TL;DR, it's here:

This is Geopolitics 101: great powers are always sensitive to potential threats near their home territory. After all, the United States does not tolerate distant great powers deploying military forces anywhere in the Western Hemisphere, much less on its borders. Imagine the outrage in Washington if China built an impressive military alliance and tried to include Canada and Mexico in it. Logic aside, Russian leaders have told their Western counterparts on many occasions that they consider NATO expansion into Georgia and Ukraine unacceptable, along with any effort to turn those countries against Russia—a message that the 2008 Russian-Georgian war also made crystal clear.

This whole article is worth a read (and considering my historical anti-reading stance this is a strong endorsement). It lays out the entire post-Soviet history of how NATO has regularly antagonized Russia in ways that no NATO member state would ever find acceptable, and how Russia has always responded militarily.