The seeming volte-face of the Entente had several origins. Firstly, the Bolshevik Revolution greatly scared the Allies and they feared that the Romania's incursions destabilized the region greatly. The advent of Bela Kun and the expansion of the Russian Civil War seemed to confirm these suspicions. Although by this point most Allied leaders had realistically given up on direct intervention against the Bolsheviks, they desired to erect in Eastern Europe a cordon against Communist expansion. Marshal Foch would say he "preferred a barbed-wire entanglement around Russia." Especially for the British foreign office, Allied leaders felt that Romania territorial gains were at the expense of the long-term interests of the European continent. The Romanian occupation of Budapest was for them an annexation too far; Romania had already gained considerable Hungarian territories. The man arguably most responsible for swaying Allied opinion against its Romanian ally was Sir George Russel Clerk. Primarily a specialist of Czechs, Clerk forged a series of personal connections with the post-Hapsburg elites in the successor states. Among his connections was Miklos Horthy. Horthy gave the Clerk two promises: to end the Red menace and restore stability to Hungary. This dovetailed nicely with the French desire for a "little"-Entente in Eastern Europe the British Foreign Office's desire to restore the European economy by bringing peace to the Danube basin. The Romanian occupation ran counter to these goals.

sources

Protheroe, Gerald. "Sir George Clerk and the Struggle for British Influence in Central Europe, 1919-26". Diplomacy & Statecraft. 12, no. 3: 39-64.

Sugar, Peter F., Péter Hanák, and Tibor Frank. A History of Hungary. Bloomington: Indiana University Press, 1990.