In the past, that line has been drawn according to whether the USA wants it to happen or not.
But, in the eyes of international law and the UN's charter, "All peoples have the right of self-determination. By virtue of that right they freely determine their political status and freely pursue their economic, social and cultural development."
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Crimea's democratically-elected parliament recalled this chapter of the UN's charter when they declared their then-upcoming 2014 referendum.
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The idea of self-determination is that people aren't the property of other people, but start out with a natural right to freedom - and so it isn't for others to say what a group of people chooses for their future.
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If you believe that a peoples' right to self-determination in subject to the approval of others, then you don't believe in a right to self-determination, at all, and instead believe that some people are the subjects of other people, and therefore are not equal.