In brief, Ukraine has several ethnicities and one of them are Russians. Combine that with Russia not having a good land connection with their most important naval bases in the Black Sea, the Crimean Peninsula, this has given the President of Russia - Putin - a reason to wage war on Ukraine. He will most surely not be satisfied until the east of Ukraine is Russian, absorbing the Russian minority and the land-bridge.
WHat is happening right now is that Russia is funding and aiding rebels in the Ukraine, comprised mostly of ethnic Russians, who want that region of the Ukraine to become part of Russia. Russian Black-Ops have worked inside of the Ukraine with guns in their hands, and supposed “russian soldiers on leave” volunteered to go into the Ukraine and fight with the rebels.
Now however, and a few weeks back, Russia has not just sent armoured fighting vehicles to aid the rebels, but actually went over the border with their military fighting against Ukrainian forces. And the other day Russian military forces engaged in heavy fighting inside the Ukrainian border, and that constitutes by any definition as an invasion and a declaration of war.
Europe talks about economic sanctions against Russia - and some have been done - but nothing serious because many European nations need those deals with Russia. Europe is not prepared at this time to go to Ukraine’s defense for many reasons.
So, in short, Putin is doing a repeat of Hitler in 1938 when he said that the Czechoslovakian land bordering to Germany should be German because it has a German minority being oppressed and nobody is stopping him. Just like with Hitler.