The Carinthian Plebiscite (German: Kärntner Volksabstimmung, Slovene: Koroški plebiscit) on October 10, 1920 determined the border between Austria and the newly formed Kingdom of Serbs, Croats and Slovenes (later Yugoslavia) after World War I. In particular it divided Carinthia, formerly a province of the Austrian-Hungarian Monarchy, in two parts.

History
After the ruin of the Austro-Hungarian Monarchy in World War I, new states arose on its territory. Among these there was an internationally recognized State of Slovenes, Croats and Serbs, which was created on October 29, 1918, but was incorporated in the newly formed Kingdom of Serbs, Croats and Slovenes on December 1, 1918.
Determination of borders between the new countries was a problematic issue that was not always solved peacefully. The "Carinthian question" became an issue in the closing days of World War I. The principle of self-determination, called for by Woodrow Wilson, was taken up by the various nationalities that were going to form the successor states in the wake of the defunct Habsburg empire. Events in Carinthia began to unfold rapidly, beginning with territorial claims by the Slovenian National Assembly on October 17, 1918. These claims where rejected by the Provisional Provincial Carinthian Assembly on October 25, 1918. On November 11, 1918 the Provincial Carinthian Assembly demands self-determination, which in this case amounted to demanding a plebiscite, for a region with a mixed population. The question was whether the strong Slovene majority in the province's southern region adjoining the Karawanken frontier would carry the vote for union with Austria or whether they mainly wished to join the newly arisen South Slavic state. This was to large extent a consequence of rising national awareness under the multi-national Austrian-Hungarian Monarchy and dreams of autonomy, which Slovenians had not experienced since the 9th century, when the principality of Karantania lost its autonomy. A common state with other south Slavic nations seemed at that time the most acceptable compromise towards fulfillment of national strivings.
With the occupation of Lower Carinthia by Yugoslavs troops the conflicts evolved into clashes of arms. The fight to preserve the Karawanken frontier began. A nine-day American commission, the "Miles mission," scouted the disputed region between the river and the mountains in January/February 1919 and made the crucial recommendation that the Karawanken frontier should be retained, and thereby opened up the possibility of a plebiscite. The Yugoslavs pressed for a border on the Drava; the U.S. delegates spoke in favor of the preservation of the unity of the Klagenfurt Basin and succeeded in convincing the British and French delegations.
The winners in World War I wanted to solve conflicts peacefully and they divided Carinthia into two zones, A on the south and B on the north, with the intention of later organizing a referendum about annexation to either Austria or Kingdom of Serbs, Croats and Slovenes.
The plebiscite took place in the A zone on October 10, 1920, with 22,025 votes for Austria and 15,279 for the other option. This means that also a great number of Carinthian Slovenes must have voted for Austria. Because the Austrian side won, the referendum was not carried out in the northern zone B.
The Plebiscite did not cover the areas of Carinthia now part of Slovenia, namely the former "Miestal" and the area around Dravograd (German Unterdrauburg). These were likely simply annexed by the SHS kingdom as a consequence of the Treaty of Saint-Germain.
The plebiscite determined the border between Austria and the Kingdom of Serbs, Croats and Slovenes. The border remained unchanged after World War II, when the Kingdom of Serbs, Croats and Slovenes was succeeded by Tito's Socialist Federal Republic of Yugoslavia. Since the downfall of Yugoslavia, the border has separated Austria and Slovenia.