To President Trump’s offer to buy Greenland, the Danish prime minister responded “absurd.” After all, Greenland is under the sovereignty of Denmark. But how did Denmark acquire dominion over all of Greenland?

Europeans first encountered Greenland in the year 900 when Eric the Red, an Icelander of Norwegian origin, arrived on the southwest coast. By 1500, the Nordics had disappeared. They were replaced by Inuit people traveling from North America, and today Greenland is 90 percent Inuit. But in the language of the international law of the time, Greenland was “terra nullius” – land belonging to no one.In 1380 Denmark and Norway were united under one crown and the king claimed sovereignty over the territory long after the Nordics disappeared.

After the union of Norway and Denmark was dissolved, the 1814 Treaty of Kiel conceded certain territory to Norway, with the exception of the Faroe Islands, Iceland and Greenland. The Danes assumed that their sovereignty applied to the entire island. In 1909 Robert Peary, Matthew Henson and four Inuit men – Oatah, Egingwah, Seegloo and Ookeah – reached the North Pole. The age of Arctic exploration was spurring European and American interest in the High North. And Norway had commercial interests in Eastern Greenland.

During the Great War of 1914-1918, Denmark ceded its West Indian islands to the United States under the Antilles Treaty. The U.S. government declared it would not object to the Danish government extending political and economic interests to all of Greenland. Denmark then placed the matter of its sovereignty before a committee of the 1919 Paris Peace Conference. On July 22, 1919, Norwegian Foreign Minister Nils Claus Ihlen told his Danish counterpart that “the Norwegian Government would not make any difficulties in the settlement of this question.” Whereby the Danish minister reported to his government that “the plans of the Royal Danish Government respecting Danish sovereignty over the whole of Greenland would meet with no difficulties on the part of Norway.”

But in the opinion of Oslo, the non-colonized areas of East Greenland were terra nullius and if that situation changed, then that territory must revert to Norway. Negotiations failed to settle the matter and the dispute was submitted to the Permanent Court of International Justice established by the League of Nations. In the 1933 case of the Legal Status of Eastern Greenland, the court concluded that Mr. Ihlen’s words were “binding upon the country to which the Minister belongs.” Hence, Norway was under a legal obligation to refrain from contesting Danish sovereignty over all of Greenland.

During World War II there was a key American airbase at Narsarsuaq near the Eric the Red Settlement, and by the early Cold War, another at Thule in the north. With geo-strategy in mind, President Harry Truman made Denmark an offer to purchase Greenland in 1946. Today, the world’s largest island still remains in the Danish Realm and Denmark retains competence over its foreign affairs. But in 1979, Denmark granted home rule to Greenland, and the Self-Government Act was implemented in 2009. In the capital at Nuuk, there is a Department of Independence, and a Constitutional Commission has been at work. So if President Trump seriously wished to purchase Greenland, should he not consult the real party in interest?

Former Secretary of State Dean Rusk wrote, “One evening over a highball or two, I suggested to the Foreign Minister of Honduras that we toss a coin for Swan Islands (claimed by both countries). Fortunately he refused, because the Permanent Court of International Justice in the Greenland case seemed to say that a government has the right to rely on the statement of a Foreign Minister in a territorial matter.” But would territory transferred by a coin toss be more or less absurd than transfer by a sale that accounts for the people who live in the territory?