>Why should European states assume the US has an obligation to always support Europe?

Well, it agreed to:

>In 1994, Ukraine agreed to transfer these weapons to Russia and became a party to the Treaty on the Non-Proliferation of Nuclear Weapons, in exchange for assurances from Russia, the United States and United Kingdom to respect the Ukrainian independence and sovereignty in the existing borders.

I don't necessarily disgree with the central thesis of your comment and your perspectices, but i think there is a more fruitful balance to be had than what i see as completee capitulation to Russia and the abandonment of Europe (after almost a century of collaboration and investment).

At some point one need to ask oneself: what am i defending?