Putin had troops amassed on Ukraines border for around 6 weeks with no response from the US or EU and a past history of allowing the Crimean incursion and takeover. And then that Green Light message from. Biden, AND the message from Biden that the US is going to end Trump’s ban on helping Russia complete their Nordstream II pipeline under the Black Sea to allow making even MORE money selling overly dependent Europe it’s oil while dismantling Trumps efforts to try to provide Europe its needs instead.
Putin expected no resistance at all. The US leader had already capitulated in the past, and Europe needed its oil to heat homes in the winter.
And he would have gotten none of Ukraine had surprised everyone by hanging on for so long.