A big part of the problem in terms of convincing European leaders to side with the United States and Ukraine against Russia is the relationship of the Ukrainian government (or at least the Yatsenyuk government which preceded the current Poroshenko government) with Ukrainian neo-Nazis such as Pravy Sektor. This makes Germany and France pretty reluctant, as well as being somewhat of a concern for at least some Poles. Whilst Poland is pretty anti-Russia, I actually knew an older Pole who was hoping Russia would win, because the village his family were from was burnt to the ground by the Ukrainian Insurgent Army at the end of WWII.
In the event that a war actually happens, I don't think we can be sure that the Baltic States would jump in. As much as Lithuania, Latvia and Estonia would side with the United States diplomatically, few countries are willing to make their country a battlefield, especially small ones. Not to mention that Latvia has a very large Russian minority which can't be sufficiently watched over, and Russia has already shown the kind of damage it can do to Estonia using cyberwarfare alone.
Poland would be likely to send volunteer contingents, given that their territory is unlikely to be directly affected assuming that the Americans can maintain defensive air superiority. Belarus probably won't be a route for a Russian attack on Poland since it is in the Russian interest to keep the war small, controlled and localised, with suits Lukashenko just fine, since he spends most of his time maintaining his power anyway.
I read an interesting article by The Diplomat yesterday, which pointed out aspects of the post-2008 Russian military reforms which have gone largely ignored by international observers. They're oriented largely towards improving professionalism and whilst incomplete, look like they're showing a lot of promise. We can expect a much more competent Russian performance than in Georgia.
We also shouldn't forget that Russian tactics have an effective dual orientation. The first part of this is that they're designed to counteract American superiority in firepower. The United States may maintain air superiority over Ukraine, but they can't achieve air supremacy, which most of their offensive doctrine relies upon. Secondly, Russian tactics are constructed with an eye on publicity and minimising political fallout, which is under-appreciated when looking at purely military factors.
Russian intervention would likely brush aside the Ukrainian army if it seeks to engage them immediately upon entering Ukraine. The remnants of the Ukrainian army fall back to the Psel at least and probably the Dnieper. By this time the Americans are likely to start deploying via the Black Sea. This will involve sending a Carrier Battle Group to cover deployment whilst the Western Europeans drag their feet. This Group is likely to be engaged by a mixed force of the Russian Black Sea Fleet, land-based aircraft and cruise missiles. The outcome isn't 100%, and this could impact America's domestic political landscape quite a lot, dictating whether escalation occurs. Nevertheless, the United States has to neutralise Sevastopol or they will be unable to win any real victory in Ukraine.
Kiev would be a meat-grinder for the Russians and they would know it. It's at this time that they would seek a settlement which would probably involve the fall of the government in Kiev and its replacement with a "National Unity Government" combining moderates, pro-Russian politicians and Donetsk separatists. It would be a Russian puppet essentially, even more so than Yanukhovych. Whether the Americans accept such terms or not depends on the particular leadership and internal political situation.