Conquering any of the Balkan countries should run afoul of several major countries' interests. Italy had designs on the area, and would have intervened, either to oppose a grab by anyone else, or at least to take their own "share" of any of the countries targeted. France strongly supported Yugoslavia and Czechoslovakia, and Romania to a lesser degree, as insurance against the resurgence of an Austro-Hungarian power bloc without being any threat to anyone else, and would have taken action to prevent any one of them from uniting the region into a real "player" in the area. Romania, while part of the "Little Entente", would also have had to deal with both of its "partners" in the matter in some manner. The Soviets were also beginning to take a more serious interest, were actively fomenting rebellions in Macedonia and Croatia, and might have "offered to assist" any country being invaded.
All of the major powers had an interest in preventing any other bloc from gaining an upper hand, and any disturbance of the "status quo", especially in the Balkans, was already being vigorously opposed by nearly all parties. The Anschluss of Austria by Germany only became possible due to a sudden shift in the balance and an assurance that Italy would not go to war over it.