President Trump is upset that Danish Prime Minister Mette Frederiksen called his interest in purchasing Greenland "absurd." Her dismissive response should have come as no surprise. In 1946, when President Harry Truman tried to purchase Greenland, Secretary of State James Byrnes wrote that the proposal "seemed to come as a shock" and an insult to Danish officials, who turned it down.

That was a big mistake. As part of his deal, Truman had offered to trade parts of the Point Barrow district of Alaska, including the rights to any oil discovered there, to Denmark, in exchange for parts of Greenland. The Danes dismissed the idea just as they did Trump's proposal. In 1967, the richest oil strike in U.S. history was made in the Point Barrow area. Bad move, Denmark! Sad!

With that blunder in their rearview mirror, you would think that Danish leaders would at least hear Trump out. The president's idea of buying Greenland is far from absurd. Today we have a military base in Greenland, so there is no need to buy it for that purpose. But Greenland has enormous unexplored stores of natural resources, including zinc, lead, gold, iron ore, diamonds, copper and uranium, that Denmark has been unable or unwilling to exploit.

It also has large, untapped stores of rare-earth elements, such as praseodymium or dysprosium, that are critical to the production of everything from electric cars to smartphones and lasers. Today, the United States gets many of these rare-earth elements from China, which makes Americans dependent on Beijing. The Wall Street Journal reports that Beijing may cut off access to those minerals in its trade dispute with Washington, and China is also trying to corner the market for rare-earth elements in Greenland. Buying Greenland would put those strategically valuable minerals in U.S. hands.

But what makes Greenland particularly valuable to the United States is global warming. The unavoidable receding of Arctic sea ice will open a new sea route in the Arctic that can be used for both commercial and military vessels. In May, Secretary of State Mike Pompeo delivered an address at the Arctic Council Ministerial Meeting in Finland in which he pointed out that "steady reductions in sea ice are opening new passageways and new opportunities for trade. This could potentially slash the time it takes to travel between Asia and the West by as much as 20 days." He added that the emerging "Arctic sea lanes could become the 21st-century Suez and Panama canals."

He's right. A recent report in the New York Times notes that as sea ice melts and "Arctic routes become more direct, voyage times could fall to less than three weeks in some cases, making Arctic shipping potentially more attractive than the southern routes in coming decades."

The United States and its allies have a major interest in not allowing these Arctic sea lanes to fall under Russian or Chinese control. "Do we want the Arctic Ocean to transform into a New South China Sea, fraught with militarization and competing territorial claims?" Pompeo asked in Finland. Purchasing Greenland would help the United States to better secure these emerging strategic passageways.

Rather than getting offended, Copenhagen should entertain Trump's offer. In 1916, it sold the Danish West Indies (now the U.S. Virgin Islands) to President Woodrow Wilson. So, we've long ago established that parts of Denmark are for sale; there's no harm haggling over the price.

Marc A. Thiessen writes a twice-weekly column for The Post on foreign and domestic policy.