[jemmyw]

The US, UK and Russia did have a security guarantee with Ukraine in return for them giving up the nuclear weapons they had under the Soviet union.

Ukraine has strongly indicated they want to join the EU and NATO and have moved towards a more democratic government (with corruption problems, but some western allies also have corruption). Self serving as that might be they are, for all intents and purposes, our allies. And they are rather strategically positioned.

And then we also don't like one country systematically attacking the civilians and civilian infrastructure of another country. There are various agreements about rules of engagement, not annexing within agreed upon borders etc.

In answer to your question then: nothing, there's nothing special about Ukraine. That's the point.