In late 1918 Romanian leaders traveled to Paris to forward the kingdom's broad territorial claims at the upcoming peace conference, which opened on January 18, 1919. At the conference, Romania insisted that the Allies respect the principle of national self-determination and fulfill the territorial promises made in 1916 that had brought Romania into the war on the side of the Allies. The Allies had promised Romania the Banat (see Glossary), a fertile agricultural region bounded by the Tisza, Mures, and Danube rivers, which Serbia also claimed because of the region's large Slavic population. The conference participants supported almost all of Romania's claims, including those to Transylvania, Bessarabia, and northern Bukovina, but arbiters finally partitioned the Banat between Romania and Serbia.

In March 1919, the French head of the Entente mission in Budapest handed Mihály Károlyi, the fledgling Hungarian republic's leftist president, a diplomatic note dictating the last in a series of border rectifications that stripped Hungary of large swaths of its traditional lands. Károlyi resigned in disgust and turned power over to a coalition of social democrats and communists, who promised that the Soviet Union would help Hungary restore its prewar borders. The communists, under Béla Kun, immediately seized control and announced the founding of the Hungarian Soviet Republic. In late May, Kun backed his promises to restore Hungary's lost territories with military action against Czechoslovakia. When the French threatened to retaliate, Kun turned his army on Romania. Romanian units, however, penetrated Hungarian lines on July 30, occupied and looted Budapest, and scattered the members of Kun's government. When the Romanian troops finally departed Budapest at the beginning of 1920, they took extensive booty, including food, trucks, locomotives and railroad cars, and factory equipment, in revenge for the Central Powers' plundering of Romania during the war.

Romania's occupation of Budapest deepened ongoing Hungarian bitterness at the Paris conference against Bratianu, who stubbornly opposed the partition of the Banat and provisions of the treaties guaranteeing rights of minority ethnic groups. When Bratianu resigned rather than accept the treaty with Austria, King Ferdinand appointed a nonpartisan government and called for elections. In 1919 Romanians voted in the country's first free elections and swept away the Liberals' artificial parliamentary majority. Victory went to Iuliu Maniu's National Party, the major prewar Romanian party in Transylvania, which quickly carved out a niche in the political life of Greater Romania (see Glossary) by attracting peasant support in the Old Kingdom, the territories of pre-World War I Romania. Maniu's colleague, Alexandru Vaida-Voevod, became premier and rapidly signed the treaties. Vaida-Voevod ran the government until 1920, when the king named General Alexandru Averescu premier.

Data as of July 1989