Putin feels neglected
The U.S. has been engaged with China for some time regarding China’s threats in the South China Sea and Taiwan ( https://apnews.com/article/joe-biden-china-united-states-beijing-united-nations-0f27ca946b9fa97487af1ba8e80c3682 ).
This apparently has made Russian President Vladimir Putin feel a bit neglected on the geopolitical stage so, consequently, he has amassed a significant World War Two vintage military presence along Russia’s border with Ukraine.
( https://www.cnn.com/2022/01/28/politics/us-russia-ukraine-invasion-warning/index.html )