In its initial phases in the Middle Ages, the essence of feudalism was a contract: the ownership of land in exchange for a specified amount of military assistance. Before long, this morphed into a slightly more flexible relationship: services of a variety of kinds and complete loyalty in exchange for protection. That’s how the mob works today. Not coincidently, it is at the heart of Trump’s view of foreign policy.
Ukraine, facing an enormous deficit in manpower and resources, is eager to be Trump’s vassal. But unlike, say, Canada or Mexico, Trump doesn’t want Ukraine as a vassal state; he wants an alliance with Putin, and Ukraine stands in the way. That is why he is so eager to make peace; he doesn’t want to fulfil his side of the feudal contract.