As Dylan says, the USA - among others - has treaty obligations to guarantee Ukraine's territorial integrity. This is not a trivial issue. The undertaking involved, among other things, Ukraine giving up its nuclear arsenal. To demonstrate that such treaties will not be underwritten knocks a huge hole in the post-WW2 international legal framework, such as it is, and in the reputation of the USA and other actors as treaty partners.

It's a ticket to a world of uncontrolled nuclear proliferation.