US President-elect Donald Trump has once again expressed an interest in buying Greenland, the world’s biggest island and a self-ruling territory of Denmark.

Trump already put forward the idea in 2019, during his first term as president. Both then and now, the response from the other side of the imagined transaction has been a resounding “no.” And it’s still not clear whether such a deal is even feasible.

Why would Trump want to buy Greenland?

As Trump surely knows, other US leaders have made their mark in history for such grand ideas, with Andrew Johnson remembered as the president who oversaw the acquisition of Alaska from Russia in 1867.

When Trump suggested a potential Greenland purchase back in 2019, he presented it as “a large real estate deal” that could ease Denmark’s state finances.

His main argument this time around is that US control of the island is vital for national security.

What interest does the US have in Greenland?

Lying in the northern Atlantic Ocean between Europe and America, the island has long been a nexus of tensions among global powers. Besides being bigger than Mexico and Saudi Arabia, Greenland has a strategic location straddling the North Atlantic and the Arctic, a region whose vast stores of critical minerals and fossil fuels are coveted by the US, Russia and China. It’s already home to the US’s northernmost air base and a radar station that’s used for detecting missile threats and monitoring space.

How credible is the idea of the US buying Greenland?

Until a century ago, these kinds of transactions were not unusual: Before the Alaska purchase, the US bought the territory of Louisiana from France for $15 million in 1803. There’s even a precedent involving Denmark. The government in Copenhagen sold what are now known as the US Virgin Islands in 1917. There are in fact plenty of other islands for sale, but they tend to be smaller and located in warmer climates. Joseph Blocher of the Duke University School of Law wrote in 2014 that the “market for sovereign territory seems to have dried up.”

How much would Greenland cost to buy?

Since the 830,000 square-mile (2.1 million square-kilometer) island isn’t currently on the market, there are no valuations available. However, for a sense of how much it might fetch, it’s worth noting that its gross domestic product totaled just over $3.2 billion in 2021, according to the World Bank. The island receives an annual subsidy of around $600 million from Denmark. For perspective, the purchase price of Alaska in 1867, $7.2 million, would translate to just over $150 million today — which most would agree would be quite a bargain.

How would Trump go about buying Greenland?

It’s unclear. When he brought up the idea in 2019, adjunct Professor Rasmus Leander Nielsen of Greenland University told local media that Denmark can’t sell Greenland because its home-rule law of 2009 “clearly states that Greenlanders are their own people.” Trump’s best hope would be for the territory to gain independence and then choose to arrange a sale to the US. As it turns out, breaking away from Denmark has long been discussed in Greenland, and last year local lawmakers unveiled a draft constitution for an independent nation — though full independence may still be some time away for the island of 57,000 people.

Greenland Prime Minister Mute Bourup Egede said in response to Trump’s latest suggestion that the island “will never be for sale,” although the territory was still open to cooperating and trading more with the rest of the world, especially its neighbors.

Earlier this year, Greenland’s autonomous authority published a foreign, security and defense policy blueprint that expressed a desire to forge closer links with North America through trade in critical minerals, and by Greenlanders having a bigger say in key defense matters that have historically been overseen by Denmark.

Could Trump do this on his own?

Under the US Constitution, Congress would have to approve money for any such purchase. But Trump has previously shown a willingness to try to go around Congress’s power of the purse. During his first term, when Congress refused to appropriate money for more fencing on the US-Mexico border, Trump raided the Pentagon’s budget to find funds for it — and the Supreme Court backed him up.