The Biden Administration knew this outcome was going to happen. They even predicted that Russian aggression into Ukraine was unfolding exactly the way it is currently playing out. So why was nothing done about it? The first part of that answer is that the United States Government missed their first opportunity all the way back in 2015 under Barack Obama. John J. Mearsheimer spoke about the precise ways in which Putin can be stopped incrementally and weaken his positions before he can shore them up much later and be better prepared.
Now the next question is why did Trump not listen to this advice after he got into office? The plan of deterrence under the leadership of Donald Trump worked with Vladimir Putin. Regardless of the way it was done it got done. Putin and Trump respected each other's positions on the global stage and had a mutually beneficial relationship. The biggest proponent of why this did not happen under Trump is that we were energy independent. We did not rely on foreign oil as much as we do now.
Biden took it upon himself to make us energy dependent again based on his energy policy. What he did not count on was that his energy policy also impacted foreign policy which he has a terrible record on. The chickens have come home to roost based on the decisions of our failed leadership and now our actions have more bark than bite.