There was a referendum in Northern Ireland in 1973 as to whether it should remain part of the United Kingdom or join the Republic of Ireland.

The exclave of Campione d'Italia petitioned for cession into Switzerland, as did Busingen; the Swiss declined in both cases. There was also the East Prussian plebiscite, which decided the fates of the regions Warmia, Masuria, and Powisle- whether they were to stay with Prussia, or become part of Poland.

Robinson, G.W.S. "Exclaves." Annals of the Association of American Geographers 49, no. 3 (September, 1959): 283-295.

Wambaugh, Sarah. Plebiscites Since the First World War: With a Collection of Official Documents. Oakland: University of California, 1933.

/u/ctesibius mentioned the Northern Ireland referendum. There was also very recently the Scottish referendum on the same question. Though it was a much closer race than the NI one it had the same result and the UK remains intact for now.

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