A communist Balkans before the First World War

How would Europe react if Serbia, Romania, Bulgaria, and Greece became communist after the Second Balkan War?

The major powers would likely mobilize or intervene together because they would see a communist Balkan bloc in 1913 as a threat to every existing regime.

The answer expects rapid intervention or mobilization by powers that otherwise distrusted one another, including Britain, France, Germany, Russia, and possibly the Ottoman Empire. In 1913, revolutionary communism across Serbia, Romania, Bulgaria, and Greece would appear as a shared systemic threat rather than an ordinary local realignment. The scenario therefore creates a brief basis for great-power cooperation even as Europe approaches the First World War.