President-elect Donald Trump won’t rule out use of the U.S. military to take control of the Panama Canal and Greenland, if necessary.

Trump, during a press conference held Tuesday, said he is “not going to commit to that,” when asked to rule out use of American troops to secure either territory.

“It might be that you’ll have to do something. The Panama Canal is vital to our country” he said, adding that, “we need Greenland for national security purposes.”

Trump’s comments come as his son, Donald Trump Jr., is in Greenland on an unofficial visit. Ahead of the visit, Trump said via his Truth Social media platform on Monday evening that he’s “hearing that the people of Greenland are ‘MAGA.’”

“Greenland is an incredible place, and the people will benefit tremendously if, and when, it becomes part of our Nation. We will protect it, and cherish it, from a very vicious outside World,” he wrote, capitalizations his.

Greenland is currently an autonomous territory of NATO member Denmark. The island is home to about 60,000 residents and has its own Prime Minister and parliament. When Trump first floated the idea of buying Greenland in 2019, Denmark’s Prime Minister Mette Frederiksen, called the proposal “absurd,” and during an interview on Danish TV aired Tuesday she said that the island “belongs to the Greenlanders.”

However, this is not the first time the U.S. government has considered making the island an American territory.

According to Maye Henning, a professor of politics and legal studies at Suffolk University, talk of acquiring Greenland occurred as early as the 1860s, when then-Secretary of State William Seward was considering the acquisition of Alaska from Russia. The subject came up again in 1917, when the U.S. did acquire territory from Denmark in buying the Virgin Islands.

“There is precedent for this idea being floated,” she said.

Despite that, Henning said, Trump’s comments can seem “very out of pocket,” considering that for most of the last century the U.S. has avoided territorial expansion in favor of allowing local populations to self-determine their own future.

Beyond that, Henning said, the U.S. hasn’t changed its territorial boundaries for eight decades, since it acquired the Marina Islands.

“There are strong norms relating to the fixity of territory, and also there are strong norms liberal democracy about consent of the governed. Denmark made it very clear that they are not willing to sell Greenland and my understanding is that residents of Greenland are very opposed to coming under U.S. sovereignty,” she said.

Outside of buying the island or convincing it’s citizens to join the U.S., Henning said the only way Trump gets what he wants is “through military action.”

“Through invasion, basically, and war to acquire Greenland. That would be rather unprecedented in the modern era,” she said.

There are strategic reasons for the U.S. to consider acquiring both the Panama Canal and Greenland, Henning said.

Climate change is melting the glaciers on Greenland, potentially opening that land for development and resource extraction while simultaneously clearing new shipping routes, she said. At the same time, the lake that feeds the Panama Canal’s locks is drying up amid prolonged drought, reducing the number of ships that can transit through in a day.

“Trump and other leaders see this arctic region as an alternative to the Panama Canal,” she said.

Whether Trump will continue through with his plans remains to be seen, the professor said. In the case of Greenland, there is also an open question over whether he would even need to take the island. The U.S. already foots the bill for defending Greenland and already has a sizable military presence there, she said.

“Why purchase the entirety of Greenland or go to war over it when we already have existing legal options for operations such as treaties or leasing lands?” she asked.