In the world sphere, there is no de facto government. The UN is an attempt to create some sort of world governance, but as we know it's quite imperfect and it doesn't have the stick (i.e., the equivalent of a police force) to enforce its resolutions. Thus, the world works in a fragile equilibrium of military and economic power, with multiple countries forging alliances and pacts to try to put in place some form of civility.
So, while Ukraine "can decide" what alliances to join, there is nobody to ensure that it "can decide". Ukraine was relying on the West/US/Nato to get some say in its future, but we are seeing how that's unfolding. Ukraine unfortunately has to "earn" this right by forging alliances or building a stick big enough to ensure that others respect it.
Ukraine doesn't have a stick (it gave it up after the fall of the Soviet Union) but Russia does. Thus, Russia gets away with enforcing its own view of the world.
Unless other countries use their own sticks to enforce their point of view about what should happen with Ukraine, Russia will simply get away with it. "Rights" have very little to do with will happen.
I cannot think of a time in human history when things didn't work like this.