President Donald Trump’s interest in buying Greenland stirred up derision at first — and then controversy over his response to Denmark’s declaration that the massive island is not for sale. But Trump, who made his bones doing Manhattan real-estate deals, knows a promising piece of land when he sees it.

Last year, when we discussed the failure of some NATO allies to live up to their obligations, we primarily focused on Canada and Germany, both of which had some very visible failures. But the Danish government could also been cited — since the country isn’t even at 1.25% of GDP when it comes to defense spending since 2012.

This is not to say that Denmark doesn’t have good gear; its Absalon-class multi-role support ships and Ivar Huitfeldt-class guided-missile frigates, with their versatile StanFlex mission modules, are actually far superior to the U.S. Navy’s littoral combat ships.

Still, the Danes don’t spend enough. Greenland is strategically vital in terms of access to the Arctic and Atlantic. During the Cold War, it anchored one end of the Greenland-Iceland-UK gap. Control of this gap would’ve helped NATO whittle down the Soviet Navy’s armada of submarines that were intended to attack convoys bringing supplies to Europe in the event of World War III.

Today, Greenland’s importance is as much as a staging base for operations in the Arctic as it is for helping keep the Russian Navy out of the Atlantic. And that’s before one looks at its vast natural resources, including oil and uranium, both of which are key to energy independence — and thus weaken Russia’s geopolitical position while strengthening ours.

In short, Greenland is a valuable asset, and it should be defended. But in addition to its three Ivar Huitfeldt-class frigates and two Absalon-class support ships, Denmark has but seven offshore patrol vessels and six smaller ones — not many hulls to defend a lot of coastline. The Danes also have 30 F-16 Fighting Falcons (slated to be replaced by 27 F-35s), four business jets modified for the maritime patrol mission, and nine MH-60R Seahawk helicopters. But again, that’s not a whole lot — and the country has commitments beyond the planet’s biggest island.

Greenland, then, is potentially vulnerable because it’s poorly defended. Just as the Royal Navy and the state of America’s carrier force has shown, it’s easy to trim the size of a military but much tougher to build it up — not to mention restoring capabilities that have been lost.

Russia, for its part, may be having trouble rebuilding, but Denmark needs more than it can muster. Selling Greenland would lessen what it needs to cover, and such a sale could serve as an indulgence of sorts for its lack of defense spending in recent years.

For the U.S., buying Greenland would be the modern equivalent to its 19th-century acquisitions of Alaska and the Louisiana Territory. And defending Greenland would be relatively easy, mostly by reinforcing our Thule Air Base, on Greenland’s northwest coast , and basing maritime patrol planes in the southern part of the island. In addition, it gives the U.S. an Alaska in the Atlantic, with all the economic benefits — to say nothing of a great strategic check on Russia.

In short, Greenland is a good buy, and President Trump is smart to consider it.