There is also the European perspective. The cornerstone of European piece policy has been the integrity of state borders. This is now violated, so this is of very grave concern.
Putin himself stated yesterday, that he considers the Ukraine a part of Russia, so there is concern that he won't stop anytime soon, unless stopped. Ironically, an occupation of the Ukraine makes him what he claimed to try to prevent - a direct neighbor to the NATO states. We can only hope he stops before attacking NATO.