President Donald Trump reaffirmed his commitment to the notion of an American “Space Force” on Monday, while addressing a meeting of the National Space Council (NSC). The proposed force would be “separate but equal” to other Armed Forces branches like the Navy, Air Force, and Army, Trump said.

“When it comes to defending America, it is not enough to merely to have an American presence in space, we must have American dominance in space,” Trump told the NSC audience. “Very importantly, I am hereby directing the Department of Defense and Pentagon to immediately begin the process necessary to establish a Space Force as the sixth branch of the Armed Forces. That’s a big statement.”

Trump first publicly spitballed about establishing a Space Force in March, when he told an audience at San Diego’s Marine Corps Air Station Miramar that "space is a warfighting domain, just like the land, air, and sea."

Still, months later, details about the function, budget, and command of the proposed branch remain vague, making it tough to predict how Trump’s vision of space as a “warfighting domain” can coexist with the longstanding international view that space should not be weaponized. These norms were established by the Outer Space Treaty of 1967, often regarded as the foundational document of space law, which outlines a basic legal framework for peaceful exploration of space, shared by all nations.

I asked Joanne Gabrynowicz, editor-in-chief emerita of The Journal of Space Law, whether establishing a Space Force would conflict with the treaty. “It depends on what the Space Force would do, what its mission would be, and where it would do it,” Gabrynowicz told me over the phone.

“The military is legal in space, and has been since day one,” she said, noting that even GPS satellites are operated by the US Air Force. “However also from day one there have been significant legal limitations on military activities.” While military equipment and services are permitted off-Earth, an ever-evolving rubric of legal protections has sustained the decades-old commitment to peaceful development of orbital habitats and other celestial worlds.

For example, Gabrynowicz pointed me to Article IV of the Outer Space Treaty, which forbids weapons of mass destruction in space and specifies that bodies like the Moon should be used “exclusively for peaceful purposes.”

Image: Outer Space Treaty, 1967

In addition to space law, Gabrynowicz noted that space is a commons—a place where humanity shares access and use, like the global oceans and Antarctica—and is governed by international law. “Because of that, there are entire bodies of law that would be applicable here,” she said.

But Trump has demonstrated with past actions, like Executive Order 13769 (aka the “Muslim ban”), that potential legal restraints are far down on his list of concerns in accomplishing an agenda. Likewise, the president has brushed aside objections from Secretary of Defense James Mattis, who opposes the proposal on the grounds that it would create “additional organizational layers at a time when we are focused on reducing overhead and integrating joint warfighting efforts," according to Mattis’ 2017 letter to Senator John McCain, chairman of the Committee on Armed Services.

Perhaps Trump simply recognizes the public relations value of a Space Force as an aggressive initiative that his base can rally around, similar to his proposed southern border wall. In this way, he is part of a tradition of American officials who’ve flirted with the notion of war breaking out in space.

In 1996, for instance, US General Joseph Ashy told Aviation Week and Space Technology that space warfare is “politically sensitive, but it’s going to happen.”

“Some people don’t want to hear this, and it sure isn’t in vogue, but absolutely we’re going to fight in space.” Ashy claimed. “We’re going to fight from space, and we’re going to fight into space.”

Whether these premonitions of future wars in space will develop into reality remains to be seen. Either way, Trump seems to regard it as an idea that gets good ratings.