The Entente and in particular, French prime minister Georges Clemenceau, progressively hardened their stance against Hungary, in favor of the Kingdom of Romania, the Kingdom of Serbs, Croats and Slovenes and the First Czechoslovak Republic. In order to both fulfill past promises and gain their support for the Allied intervention in the Russian Civil War. Its territory was restricted as result of the November 1918 Armistice of Belgrade and the March 1919 Vix Note. The latter led to the resignation of Count Mihály Károlyi's government and the subsequent establishment of the Hungarian Soviet Republic on 21 March. The new communist government initiated a purge of its political opponents which later came to be known as the Red Terror, all while fighting against Romania and Czechoslovakia on multiple fronts. The Romanian army achieved a decisive victory on 1 August, occupying and pillaging Budapest. The Romanians withdrew from the capital on 15 November, paving the way for Admiral Miklós Horthy's national army to seize control. Horthy's army launched its own purge of perceived supporters of the previous government. The Hungarian throne was proclaimed vacant after King Charles I of Austria fled to Switzerland. The national army then pressured the national assembly into electing Horthy as the regent of the Kingdom of Hungary on 1 March 1920.