It's not defense. Crimea is a warm water port, even if the Turks control the Dardanelles, they're never going to seriously give it up. That's Russian foreign policy 101. The other issue is that while Khrushchev did drunkenly transfer Crimea to the Ukrainian SSR, Crimeans are not Ukrainian ethnically or culturally and it's been part of Russia since the 1700s. They played a stupid game and, Russia, legitimately or not, got a region that thinks of itself as Russian more than Ukrainian back. It's similar in Eastern Ukraine and the Donbass region, there's a lot of ethnic Ukrainians but they're Orthodox and more culturally Russian. It's a mess we don't need to be in.
As a side note, the last branch of the Gothic languages, Crimean Goth, was spoken there until the 1700s and ethnically it's a mix of all kinds of things, Greeks, Alani, Goths, etc, on top of everything else. Fascinating region, but you have to be a complete dumbass, or the US State Department, to think Russia would ever let it slip away.
He's pure, Globalist evil, but he's very good at Realpolitik.