I strongly suspect that Biden thought, in as much as he does "think" at all, that the Ukraine was another Afghanistan and it was already a lost cause. He didn't want to sully his hands; after all, they were not worth any more $ to Hunter so were quite disposable. Then something strange to Biden happened; the Ukraine showed what the English call "bottle" and fought to the death for their motherland..........I think of the precedent of Stalin asking the Russian People to fight, not for Communism, but for "MOTHER RUSSIA"!!!(May be an urban myth but I have heard it repeated over the years). The Ukraine has a similar culture in that the land is integral to their identity. it may be a slav thing.
Greg