“Ukraine is central to this vision. Culturally and economically, Putin sees Ukraine as tied to Russia. Putin used his hot vax summer to publish an article about how Ukrainians and Russians “were one people — a single whole,” according to an English translation posted on the Kremlin’s website. For him, the ex-Soviet Republic is not really a sovereign state but belongs to Russia, or at least would if not for the meddling from outside forces (read: the West) that have created a “wall” between the two.
“Step by step, Ukraine was dragged into a dangerous geopolitical game aimed at turning Ukraine into a barrier between Europe and Russia, a springboard against Russia,” Putin wrote.
This issue of Ukraine being a “springboard” for military actionagainst Russia is also unacceptable to Putin. He wants to recreate a “sphere of influence” for Moscow, and Ukraine is the buffer between it and NATO. As Ukraine moves closer to the West, that buffer crumbles.
“The reason there’s a war in Ukraine has a lot to do with Russia’s perception of the post-Cold War order in Europe, this notion that Western states have been moving closer and closer to Russia’s borders, and indeed, gobbling up its natural sphere of influence,” Oliker said. “Ukraine’s the front line on that.””
…
“NATO and member states within NATO like the US and Great Britain are cooperating with Ukraine on security, they’re helping in training and reforms, and providing (or selling) military equipment. But a close partnership is not the same as membership, as it doesn’t come with the obligation of mutual defense, and the NATO countries don’t exactly want to sign themselves up for a potential war with Russia.”
https://www.vox.com/2021/12/8/22824015/russia-ukraine-troops-tensons-putin-biden-nato