was President, he and Biden supported the ouster of Ukraine's corrupt and Russia-friendly President and outraged Putin, so he struck Crimea...later, when Trump was in office, Putin no longer had to wage war to get what he wanted, as Trump was doing his bidding by trying to weaken both Ukraine and NATO......also, a war effort would force DJT's hand...either show weakness and do nothing, or support Ukraine and inflict damage on Russia..not good choices....subsequently, with Biden in office and supporting Ukraine again, Putin's only option was to invade Ukraine while assuming it would be as easy as the Crimea "Special Operation"...he obviously miscalculated.
Link: https://apnews.com/article/russia-putin-ukraine-crimea-seizure-8245aec572fb71236febfa8735c42879