the situation in Ukraine is pretty complex...
Russia wants to protect military interests, but pretents to help the russian population on Krim/Giving the people the opportunity the choose their leadership.
Ukraine was given Krim by Russia under the soviets, so Krim is not 'really' Ukrainan.
USA wants to protect the Ukranian democracy/Does not want Russian influence. They want to look like the good guys, while really only wanting the protect their own interests.
EU do not want WW3 to start(whatever that means, and which side to be on).

UN is powerless as Russia haves veto power on the security council, and Ukraine is not a full member of UN and thus not protected by UN.

Honestly the whole situation is a mess. It seems like the last few notes of the cold war, or the prelude for WW3.