The US has absolutely not signed a defensive pact with Ukraine. Its commitment in the Budapest memorandum is limited to respecting Ukraine’s sovereignty and bringing matters to UN Security Council in case that sovereignty is violated, which does nothing as long as the violating party is a permanent member. The whole document was pretty a nice political declaration. Don’t get me wrong, I think the US should provide all help possible to Ukraine, but saying that it has a defensive pact or a treaty obligation to do so is unfair.