Venezuela claims Guyana’s Essequibo region, a territory larger than Greece and rich in oil and minerals, as its own and whose future it intends to decide Sunday with a referendum.

The practical and legal implications of the vote, which among other things calls for turning Essequibo into a Venezuelan state, remain unclear, but the referendum has left area residents on edge.

The 61,600-square-mile (159,500-square-kilometer) area accounts for two-thirds of Guyana.

Yet, Venezuela has always considered Essequibo as its own because the region was within its boundaries during the Spanish colonial period, and it has long disputed the border decided by international arbitrators in 1899, when Guyana was still a British colony.

Venezuela’s commitment to pursue the territorial claim has fluctuated over the years. Its interest piqued again in 2015 when ExxonMobil announced it had found oil in commercial quantities off the Essequibo coast.

The disputed boundary was decided by arbitrators from Britain, Russia and the United States. The U.S. represented Venezuela on the panel in part because the Venezuelan government had broken off diplomatic relations with Britain.

Venezuelan officials contend the Americans and Europeans conspired to cheat their country out of the land and argue that a 1966 agreement to resolve the dispute effectively nullified the original arbitration. Guyana, the only English-speaking country in South America, maintains the initial accord is legal and binding and asked the world court in 2018 to rule it as such.