Several friends and colleagues have asked me why President Putin has threatened war against Ukraine. The answers are very simple and obvious if you think about it.
1. the Soviet Union (USSR) represented Russia as the head of a great multilingual and cultural empire. The USSR collapsed in the early 1990s when all member states left the empire to become independent nations. Putin and many of his followers clearly want to build a new empire by taking over as many of their former states as possible. Ukraine is Putin's first attempt to build an expanded Russia and regain lost prestige and humiliation.
2. Germany invaded Russia twice in the 1900s (World War I and World War II II) and inflicted huge losses on the Russians, hitting them hard. Historically, Russia always wanted a restricted area to protect itself from another attack by the West. Russia felt safe as long as it had control over Eastern European nations such as Poland and Hungary.
But now Poland and other countries like Lithuania that border Russia are full members of NATO. This frightens the Russians, who logically fear the West and want to remove NATO from its doorstep.
These two points explain why Russia (invaded) Ukraine. The whole idea is to make Russia great again. No wonder Putin and Trump got along so well.