In a speech last week at the Pentagon, Vice President Mike Pence announced that the Trump administration is working with the Department of Defense to create the United Space Command. The Administration is asking Congress for an additional $8 billion for space security systems over the next five years, and signaling that it is ready to work with Congress to create a sixth branch of the armed forces—the U.S. Space Force.

The idea of a Space Force, first floated by the President last March, has drawn mixed reactions from politicians and left some space advocates perplexed. Do we not want to keep outer space a domain where nations peacefully gather to conduct scientific exploration and deploy services such as weather monitoring? Isn’t that what the Outer Space Treaty of 1967 was all about?

In truth, space has always been a military arena. Many nations depend on sophisticated and expensive satellites to keep them informed and secure, and the satellites themselves require protection. The United States, Russia and China have all launched and tested anti-satellite weapons—interceptors that can destroy a satellite in space. China demonstrated as recently as 2007 how easily satellites could be damaged by an enemy.

At the same time, national security has been a technology driver for the U.S. space program since its inception, even before the Soviet Union’s launch of Sputnik in 1957 put space in the spotlight. My lifelong interest in history and the military stems in large part from being an Army brat. During my father’s military career, our family was posted at different forts around the United States and overseas. One was a special favorite of mine—Fort Huachuca, in the Huachuca Mountains of southern Arizona near the Mexican border. It was here, in the early ’60s, where my father worked on the MOBIDIC—a semi-truck-sized computer, with the then-amazing capacity to address up to seven banks of 4.1K RAM memory.

Thirty years later, in my chosen career of space science, I became involved in the DoD-NASA lunar mission called Clementine. Once again, as throughout history, science and military technology enabled each other, sparking breakthroughs in both fields. Clementine was designed to test new technologies on a space-based platform, without violating any then-existing treaty restrictions. Mapping the Moon and finding polar ice—a valuable and strategic space-based asset yet to be claimed—was a bonus. Clementine’s engineering model is currently on display at the National Air and Space Museum, in the Mary Baker Engen Restoration Hangar at the Steven F. Udvar-Hazy Center in suburban Washington, D.C.

Today the world’s economies and militaries are more dependent than ever on space assets. And with many nations, including U.S. adversaries, rapidly improving their own space technology, American satellites are increasingly vulnerable.

Scenarios for anti-satellite warfare have mostly focused on low Earth orbit (200-500 km) and geosynchronous orbit (36,000 km), where most of our satellites are stationed. Typically in these scenarios, interceptors launched from the ground either collide with an enemy satellite or explode near it.

But now that we are planning a return to the Moon, the entire volume of cislunar space (a radius of 400,000 km) becomes a potential battleground. In 1998, a launch malfunction resulted in a very valuable Hughes communications satellite being stranded in a useless orbit. Hughes engineers ingeniously used the spacecraft’s reserve attitude control fuel to gradually swing the satellite around the Moon and bring it back down to its correct orbit. The maneuver resulted in the satellite approaching Earth from an unusual, “stealthy” direction—downwards from the Moon rather than up from the ground, the way we typically think of missile launches by aggressor nations.

This “oops” event turned into an eye-opening realization for military space experts: It’s very difficult to conduct surveillance on the entire volume of space beyond low Earth orbit. China also understood the implications of cislunar orbits, and in 2010 flew its Chang’E-2 scientific spacecraft to the Moon, orbited it for a year, then moved the spacecraft to a halo orbit around the Earth-Moon L-2 libration point (60,000 km above the center of the lunar far side). After loitering there for eight months, Chang’E-2 then took off for a flyby interception of the asteroid Toutatis. In a single mission, China demonstrated “space control,” or the ability to place any kind of satellite—friendly or unfriendly—virtually anywhere in cislunar space.

Such an ability would give any nation a decided edge in a future space-based conflict. Not only does the Moon offer a gravitational rallying point for changing orbits, stealthy spacecraft could hide at the libration points of the Earth-Moon system, nearly undetectable with conventional space tracking.

It is imperative that the United States be prepared for this new theater of engagement. Building a cislunar spacefaring transportation system of the kind that I and others have advocated would put us in a position to dispatch spacecraft to any point in the Earth-Moon neighborhood, whether for science, commerce, or defense. This doesn’t mean we’re “militarizing” space—it already is militarized. And if the United States does not maintain a presence in and around cislunar space, it will have no leverage to protect its own satellites and those of our allies.

Fortunately, many of the requirements for guaranteeing national security in space are the same as those for setting up lunar outposts dedicated to science or commerce. An outpost focused on lunar ice harvesting—learning how to provision ourselves in space (it’s prohibitively expensive to bring everything along)—would serve all three purposes simultaneously. In that sense, NASA’s renewed interest in the Moon and the plan for a Space Force are in perfect harmony.