Geographically wedged between Russia & Europe, the strategically important Ukraine rarely enjoyed independence & was always ruled by foreign forces. Their last independence occurred in 1991 after USSR.
Ukraine was mostly dominated by Russia, majority either speak Russian, or are ethnic Russian. This formed a political culture where a great deal of population think of Ukraine as part of Russia & others think of Ukraine as an independent nation & resist Russian domination.
This political difference led to a great crises when in November 2013, Ukrainian president Viktor Yanukoych rejected a deal with European Union resulting in a mass protest from pro Europe group in the country. Situation worsened when the protesters were dealt with violence that divided the world into Russia backing the Ukrainian president & US-Europe supporting the protesters.
A series of events happened in same course of time when in early 2014, president was ran out by protestors. Crimea, a small piece of land in Black Sea, formerly a part of Ukraine was invaded by strange Gun men in February, 2014 who later were recognised as part of Russian armed forces. Thus Crimea which is strategically important and was fought over for years was taken by Russian military bloodlessly. Later in March, Crimean voted in favour of Russia as majority was Russian & not Ukrainian.
World sees this Crimean annexation as illegitimate. Still, call it illegitimate but Crimea has become part of Russia.
Shortly, after the annexation in April 2014, Russian backed rebels popped up in Eastern Ukraine & small fights occurred between Ukrainian forces & rebels. They captured Russian majority regions of Eastern Ukraine where Ukrainian forces posed a low profile for months because they wanted a peace deal & did not resist it with force. Later from July 2014, another turn of series of events occurred when a civilian plane of 298 passengers was shot by rebels & world finally has had enough.
Russia, in turn invaded Ukrainian borders and since then its military is pushing back the Ukraine forces. Its unclear whether Russia wants to occupy Eastern Ukraine like it did with Crimea or they are simply defending their rebels.
US & European Union are clearly outraged and are imposing sanctions on Russia but have no intention to intervene as this might spark another war, may be a world war III. So there is no stopping to Russian tanks.
Is there any end goal for Putin?
Putin arguably may not have any possible end goal with Ukraine. If he does, that would be a nationalist & populist take for his own politics or him trying to regain old Soviet dominance. Some analysts think Putin might have started the war for supporting its rebels & has immersed enough that there is no going back on that now.
What is US/EU doing?
Amidst the Ukrainian crises, west is of the view that it involves not only forced annexation and violation of basic human rights in Ukraine, but also the European see it as a threat to Europe itself. “They are coming for us next” is the threat. They are not involved directly but are imposing more sanctions on Russia to put Putin in a position to back off.
Russia is not only facing economic sanctions from US & EU but also has been alienated from G8, and facing a possible suspension from NATO along with other sanctions. All these efforts are to make Putin to pull out from Ukraine.
