AH Challenge: Join Bulgaria & Romania

Your challenge is to find a feasible way that Bulgaria and Romania would decide (or be forced to) become one country post-1900. What I mean by "one country" is open, so feel free to put them in a monarchical union or something if you think totally dissolving their borders is ASB.
 
The first Romania king Carol was proposed to be also the king of Bulgaria (pre 1900) but Russia I believed opposed.
 
Your challenge is to find a feasible way that Bulgaria and Romania would decide (or be forced to) become one country post-1900. What I mean by "one country" is open, so feel free to put them in a monarchical union or something if you think totally dissolving their borders is ASB.

Umm... this would require the elimination of national selfdetermination, as Romanian and Bulgarian are from two different language groups, even. So, any POD would have to be before the end of WWI. Even so, I don't see any way to do it.
 
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