Last August President Trump announced that he was considering making an offer to buy Greenland. But is the idea of America buying an entire other country such a ridiculous concept? In fact, in 1916 when America bought the Caribbean islands that we now know as the US Virgin Islands from Denmark the US actually agreed as part of the deal that Greenland would fall within the Kingdom of Denmark. And that's not the only example: looking back through history many countries have purchased sovereign territory. Click or scroll through for 31 other examples of nations buying all, or parts of, other nations.

Countries have been buying up sovereign territory since the medieval period. Sometime between 1097 and 1101 the Viscount of Bourges Odo Arpinus sold Bourges and Dun in what is now central France to the King of France for 60,000 pounds or shillings. Historians have suggested the viscount offloaded the territory either to fund a Holy Land crusade venture or because he had no children to bequeath it to.

Eager to expand Scotland's territory and end the nation's conflict with Norway, King Alexander III paid King Magnus IV 4,000 merks plus an annuity of 100 merks to gain sovereignty over the Firth of Clyde and Kintyre as well the Hebrides islands and the Isle of Man. The deal was formalised by the Treaty of Perth, which was signed on 2 July 1266.

The Dauphiné de Viennois, an ancient feudal state in modern-day southeastern France, was acquired by King Philip VI of France in 1349 from the heavily indebted and heirless Humbert II of Viennois, who received a lump sum of 300,000 guilders and a generous pension in return. As a result of the purchase, the heir to the French throne was thereafter known as the Dauphin ('Dolphin').

Back in 1417 the ruler of the Burgraviate of Nuremberg, an affluent territory in what is now Bavaria, Germany, parted with 400,000 Hungarian gold guilders to acquire the Margraviate of Brandenburg, which was located in modern-day northeastern Germany. The purchase paved the way for the formation of the Kingdom of Prussia. The picture shows a model of the Margraviate capital Berlin in 1480.

Russia bagged Estonia, Livonia, Ingria and southeast Finland in 1721 as part of the Treaty of Nystad, which ended the Great Northern War that had raged since 1700. Tsar Peter the Great paid King Frederick I of Sweden two million silver rubles for the territories and handed over most of Finland in return. The signing of the treaty is depicted in this etching by Peter Schenk the Younger.

Now part of the US Virgin Islands, which features later in our round-up, the Caribbean island of Saint Croix was purchased from France in 1733 by the Danish West India Company, which was acting on behalf of the state of Denmark-Norway. The company paid a total of 750,000 livres for the territory.

After the people of Corsica rebelled during the 1750s against Genoa, which had ruled the Mediterranean island since 1284, the Genoese enlisted the military assistance of the French. The revolt was quelled and Genoa was issued with a hefty bill that it couldn't pay. King Louis XV arranged the acquisition of Corsica in 1768 in lieu of the debt and it duly became a territory of France.

In 1803 the US made the first of many territorial acquisitions when the burgeoning country bought 820,000 square miles of territory from France for a bargain $15 million, the equivalent of $340 million (£259m) in today's money. The land that made up the purchase covers a vast swathe of modern-day America including Louisiana, Arkansas, Kansas, Nebraska and Montana.

The US government got out its check book again in 1819 with the purchase of Florida from Spain. Secured in the Adams-Onís Treaty, the territory encompasses present-day Florida and southern parts of Alabama, Mississippi and Louisiana. All in all, the purchase cost the US $5 million, which is just $101 million (£77m) in today's terms.

Singapore became a de facto British possession in 1819 when Stamford Raffles (pictured) signed a treaty with the Temenggong Abdu’r Rahman and the Sultan of Johor to purchase the territory for the British East India Company. The Sultan received an annuity of 5,000 Spanish dollars, while the Temenggong was awarded a yearly payment of 3,000 Spanish dollars.

Having successfully bagged the Louisiana Territory and Florida, the US government negotiated with local Native American tribes in 1836 to acquire 3,149 square miles of land in the modern-day northwest corner of Missouri. The so-named Platte Purchase cost $7,500, a paltry $207,000 (£158k) in today's money.

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As the British government tightened its grip on India, the powers that be bought Frederiksnagore (modern-day Serampore) in 1839 from Denmark and followed up the purchase with the acquisition of the fortified Danish outpost of Dansborg (Tranquebar today) in 1845. The monies paid amounted to some £125,000, which equates to $18.7 million (£14.2m) today.

In 1848 the US government signed the Treaty of Guadalupe Hidalgo with Mexico, ending the Mexican-American War. As outlined in the treaty, the US agreed to pay Mexico $15 million – the equivalent of $487 million (£371m) today – and settle the claims of American citizens against Mexico in exchange for 525,000 square miles of land spanning modern-day Arizona, California, Nevada and Utah.

Mexico sold yet more territory to the USA in 1853 in what became known as the Gadsden Purchase, which is named after James Gadsden (pictured), the US ambassador to Mexico at the time. In desperate need of hard cash, the Mexican government relinquished 29,670 square miles of territory in what is now Arizona and southwestern New Mexico for $10 million, which is $333 million (£254m) in today's money.

As part of the Gastein Convention, a treaty signed by Austria and Prussia in 1865, Austria sold the territory of Saxe-Lauenburg in present-day northern Germany to Prussia for 2.5 million Danish rigsdalers. Austria didn't hold on to the territory for long, however, losing the land a year later following the nation's expulsion from the German Confederation. The freezing-out of Austria was orchestrated by Prussian statesman Otto von Bismarck (pictured), who became Germany's first Chancellor.

Back to the US. After a period of negotiation with Russian diplomat Eduard de Stoeckl, US Secretary of State William Seward secured the purchase of Alaska in 1867 for $7.2 million, which is $125 million (£95m) today. Initially derided by some as 'Seward's Folly', the acquisition paid off handsomely when huge quantities of gold were discovered in the Last Frontier in 1896, sparking the Klondike Gold Rush. Shown here is the actual cheque used to pay for the territory.

The Dominion of Canada got in on the action with the purchase of Rupert's Land, which included modern-day Manitoba, the lion's share of Saskatchewan, southern Alberta, southern Nunavut and northern parts of Ontario and Quebec, as well as the Northwest Territories. The vendor was the Hudson's Bay Company. The sale was finalised in 1870 and cost the nation £300,000, a figure that translates to just $44 million (£33.5m) in today's money.

Exasperated with the cost of administering Saint-Barthélemy, the Swedish King Oscar II sold the Caribbean island, which is now a luxury tourist destination, to the French in 1878. In total, the purchase set the French government back 400,000 francs, which based on the price of gold at the time is around $5.2 million (£4m) today.

Not a purchase as such, the British government negotiated a 99-year lease of Hong Kong in 1898. The contract was signed in Beijing (known as Peking at the time) by China's Li Hongzhang and British diplomat Claude MacDonald, who was convinced the UK would never have to hand back the territory. Proving the British official wrong, Hong Kong of course reverted to Chinese sovereignty in 1997. A replica of the lease agreement is pictured here.

The US absorbed even more territory in 1898 following the Spanish-American War. Emerging victorious from the short-lived conflict, America was ceded Guam and Puerto Rico by Spain, granted temporary control of Cuba and gained the Philippines in exchange for $20 million, the equivalent of $618 million (£471m) today. The transfers were formalised in the Treaty of Paris, which was signed on 10 December 1898. But the Philippines did not recognise this deal, which led to the Philippine-America War. America won, but in 1935 the nation was granted Commonwealth status. After a brief period of Japanese occupation during World War II, the Philippines was granted independence by the US in 1946.

A year after giving up sovereignty of Cuba, Guam, Puerto Rico and the Philippines, the government of Spain came to the realisation it could also no longer defend and administer its territories in the Pacific. As a consequence, these islands were sold to the German government for 25 million pesetas, which is around $88 million (£67m) in today's money. This image shows the official transfer ceremony.

A number of Filipino islands were left out of the Treaty of Paris due to an administrative blunder and remained under the sovereignty of Spain. The discrepancy was addressed by the signing of a new treaty in 1900 and the territories were sold to the USA for $100,000, which works out at $3 million (£2.3m) today.

America's temporary control of Cuba came to an end in 1902. The following year, the USA took out a lease on Guantanamo Bay, renting the territory in perpetuity for an annual payment of $2,000 in gold, which is $57,000 (£43k) today, though the amount was raised to $4,085 in 1974. These days Guantanamo Bay is best-known for being the site of the notorious detention camp of the same name.

In 1903 the newly-independent nation of Panama and the USA signed the Hay-Bunau-Varilla Treaty, which granted the American government a perpetual lease on the Panama Canal and the surrounding land in exchange for a payment of $10 million and annual rent of $250,000, which is $292 million (£222m) and $7.3 million (£5.6m) in today's money respectively. In the end, the USA agreed to terminate the lease and the territory was handed back to Panama in 1999.

During the 1880s Belgium's King Leopold II, along with a syndicate of investors, gained control of the Congo from tribal rulers and went on to rule his African colony with unimaginable brutality, enslaving much of the population, who were forced to work on the territory's numerous rubber plantations. International pressure eventually forced the merciless monarch to give up the colony and it was sold to the Belgian government in 1908 for the equivalent of $63 million (£48m) in today's money.

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The most recent of America's territorial purchases, the Danish West Indies was acquired in 1916 for a total of $25 million in gold, which is $588 million ($448m) today when adjusted for inflation. The territory, which consists of three Caribbean islands, was renamed the US Virgin Islands. Interestingly, the US agreed to recognise Denmark's sovereignty of Greenland as part of the deal, something President Trump may need to be made aware of...

This territory consisting of 69 square miles in modern-day northwestern Russia near the Finnish and Norwegian borders was sold to the Soviet Union in 1947 for 700 million Finnish marks, which is $48 million (£36.6m) in today's money. The land was sold so that the Soviets could rebuild a hydroelectric plant that would benefit both nations.

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Gwadar was an overseas territory of Muscat and Oman for 200 years. A small enclave, the port city was surrounded by Pakistan and in 1958 the government of Muscat and Oman agreed to sell the territory to Pakistan for $3 million, which is $27 million (£21m) in today's money, following pressure from both local inhabitants and the Pakistani government.

The Netherlands annexed just under 27 square miles of German territory in 1949 as compensation for damage inflicted on the country during World War II. The land, which included the villages of Elten and Suderwick, remained under Dutch control until 1963 when West Germany paid out 280 million marks, $558 million (£425m) in today's money, to get it back.

The Japanese government acted in 2012 to exert its sovereignty over the Senkaku Islands, which are claimed by China and Taiwan, by buying the uninhabited territory from the Kurihara family for 2.1 billion yen, which works out at $23.6 million (£18m) today. As you might imagine, the deal didn't go down at all well with the governments of China and Taiwan, and triggered demonstrations in both countries.

The Republic of Kiribati, a sovereign state in Micronesia in the central Pacific ocean, bought land on the island of Vanua Levu from Fiji in 2014 as a potential refuge for when climate change submerges the Pacific islands that the Kiribati people currently live on. Kiribati's president Anote Tong bought 20 square kilometres of land on Vanua Levu, which lies 2,000km away from Kiribati, at a cost of $8.77 million (£6.7m), for the 110,000 Kiribati people who live on these endangered islands.

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