The British IslandsThe most notable example of an international island is in the island of Ireland. The Republic of Ireland exists on the island as an independent state and occupies five-sixth of the territory while the remaining one-sixth is Northern Ireland, a part of the United Kingdom. The partition of Ireland was determined by the governmental policy in London and the Government of Ireland Act of 1920 according to which two self-governing parts of Ireland had to exist as integrated units of the former United Kingdom of Great Britain and Ireland. With the independence of Republic of Ireland two years later the island was politically divided.
The island of Great Britain is also an example of a politically divided island even within the borders of the sovereign state of the United Kingdom of Great Britain and Northern Ireland. Wales and Scotland exists as partially autonomous entities from England with some self-governing institutions. In Scottish political life the referendum for independence is a modern and attractive idea that finds many supporters and its realization could make the Island of Great Britain split between politically independent countries.
The Baltic Sea RegionIn Northern Europe, where the Baltic Sea is surrounded by many countries, there are several examples of international sea islands. The Märket Island is one of the thousands of Aland islands between Finland and Sweden and it is separated between these two countries (in the past is was divided between Sweden and the Russian Empire). Its area is only 8.2 acres and stands as the smallest example of divided sea island in the world. These two Scandinavian countries also have another common island, the Kataja Island (Inakari in Finish, near Haparanda). It was formed by merging of two smaller islands, that belonged to both countries, due to postglacial rebound.
Finland and Russia now have parts over the Koiluoto Island in the Gulf of Finland in the Baltic Sea. In the northern part of the Szczecin Lagoon is Usedom Island, divided between Poland and Germany.
The Two Cypriot CountriesIn the East Mediterranean is Cyprus, a former British colony, now an island divided between the independent and EU member, the Republic of Cyprus (the larger part of the island) and the self-proclaimed and supported by the Turkish government, the Turkish Republic of Northern Cyprus (about one-third of the territory). The political division has existed since the Turkish invasion of 1974. Here even the capital city of Nicosia is on the border and is one urban area split between two territories. In addition, the independent British military bases of Akrotiri and Dhekelia are on the island. Their existence was part of the treaty of independence act (1960). There is also a demilitarized strip controlled by the United Nations in order to prevent future struggles between the Turkish and Greek sides. De facto, it makes the island split into four political subregions.
The Caribbean SeaThe chain-like nature of the Greater and Lesser Antilles and the colonial wars that took place on these islands determined the existence of many island-nations. Some of the islands are even divided between multiple countries.
Kiskeya is the native name of the island that Christopher Columbus renamed Hispaniola in 1492. It is one of the largest Caribbean islands and it consists of two independent states - the Dominican Republic and Haiti. Being a Spanish colony, the island was initially divided between France and Spain in 1697 due to the treaty of Riswick, and after a successful slave revolt the French part became independent in 1804. The Spanish part had more complicated trail of political sovereignty. It was independent from Spain, then occupied by Haiti, then again by Spain and it got the full independence in 1865.
There is a terrestrial border between France and the Netherlands and it does not exist in Europe but in the Caribbean Sea. The Saint Martin Island (Sint Maarten in Dutch) has been divided between France and the Netherlands since 1648. The Dutch part is one of the constituent parts of the Kingdom of the Netherlands. The French side has status of overseas collectivity.
The Island of Cuba also could be pointed out as a curious example in this list. The Guantanamo Bay military base is a part of the U.S. political space which makes the island practically divided between the Cuban state and the United States. The control over the bay was negotiated due to the Cuban-American Treaty in 1903. The current Cuban authorities do not recognize the treaty and they consider it forced and illegal. De facto, the US government controls the bay and it is part of its political space and influence.
In the Deep SouthThe Land of Fire or Tierra del Fuego is the big archipelago near the southernmost part of South America. The group of islands is separated from the mainland by the Strait of Magellan. Some of the islands are Argentinian, some are Chilean. The biggest islands, Isla Grande de Tierra del Fuego, is split between Chile and Argentina. The island itself was claimed by both countries and finally divided in 1881.
Malay ArchipelagoThe Malay Archipelago is considered the largest system of islands in the world and it is home to four international islands. The world's second biggest island, New Guinea, and the third biggest island, Borneo, (Greenland being the world's largest island) are among the divided territories. The independent Papua-New Guinea takes the eastern part of the New Guinea Island while the Indonesian provinces Papua and West Papua (known as Irian Jaya and West Irian Jaya until 2007) are on the western side. The border between them is the straight line of 141 degrees east longitude. This political division inherits the colonial division of the island. The west part was a Dutch colony which received independence in 1961 but immediately was annexed to Indonesia and it could not exist as a country. The east part was a German colony that was given to Australia after the First World War. The independence of this part was granted in 1975 and now is known as Papua-New Guinea which recognizes the British monarch as a head of state.
Three fully independent countries are located on the Borneo Island (Island of Kalimantan). These are Indonesia (73% of the island), Malaysia (26%) and Brunei. Only the State of Brunei, officially named the Abode of Peace, is entirely located on the island. It even has an exclave named Temburong. The mainland and the exclave border both with Malaysia and South China Sea. Malaysia has two states on the island (Sabah and Sarawak) and the rest of the national territory is continental or on other islands. Indonesia spreads over thousands of islands but this part here makes a big portion of its land. One of the satellite-islands of Borneo, the Sebatik Island, is also divided but 'only' between Indonesia and Malaysia.
Indonesia has one additional border island. This is the Timor Island where newly formed country East Timor (Timor-Leste) gained independence in 2002. The eastern part of the island used to be a Portuguese colony till 1975 when the land and its people was given political independence. The Indonesian government did not give any chance for a new country in the region at that time and occupied the territory causing many victims. The predominantly Catholic local people had many troubles with the Indonesian authorities and troops since the full political independence of the country under the aegis of the United Nations. East Timor has an exclave named Oecusse in the west part of the island which additionally contributes to the political diversity of the island.