What happened with Yugoslavia's president, when it happened? Why Ukraine is allowed to do exactly the same? You should understand, that this time USA has found the most painful point of pressure for Russia. It's one thing, when Syrian president asks for your help. Syria is foreign country, that is far far away. But eastern Ukraine is former Russian territories and have mostly Russian population. USA knows, that Russia would never stay away from it's population genocide. It will react. And this reaction will be treated as "aggression" and cause even more sanctions, including possible full isolation.
It's actually the same thing, that caused Russia to join WWI, while it had nothing to do with this war, that leaded to revolution and all further problems.