USA is lead by an incompetent fraud with dementia.
Putin is an old KGB agent who is motivated to rebuild Russia into an empire to the former borders of the Soviet Union.
He can not pass on the historical window of opportunity to seize on a series of military conquests and humiliations against the West.
He wants revenge for what Russia endured through the 1990's.
He also believes the West is so corrupt and weak minded that he can engage in nuclear brinkmanship. He is even willing to use nuclear weapons, and he will if he perceives his own political survival is at stake.