Trump has floated this idea before - in March, he said he initially conceived it as a joke - but has offered few details about how the Space Force would operate.
Trump said Monday that the branch would be "separate but equal" from the Air Force. Joseph F. Dunford Jr., chairman of the Joint Chiefs of Staff, would oversee its creation.
Saying that he does not want "China and other countries leading us," Trump said space was a national security issue.
The Outer Space Treaty, which the United States signed in 1967, bars states from testing weapons and establishing military bases on the moon and other celestial bodies. It also prohibits the placement of weapons of mass destruction in orbit around Earth. But the treaty has no enforcement mechanism (indeed, the Air Force's unmanned space plane, the X-37B, has completed several clandestine missions).
Trump has floated creating a Space Force for months, but the idea goes back at least a year to a proposal by Rep. Mike D. Rogers, R-Ala., and Rep. Jim Cooper, D.-Tenn. Rogers, chairman of the House Armed Services strategic forces subcommittee, and Cooper, the ranking Democrat on the subcommittee, argued that it made sense to have a "Space Corps," a separate branch of service with its own four-star general serving on the Joint Chiefs of Staff. Under their plan, it would have reported to the Department of the Air Force, in similar fashion to how the Marine Corps reports to the Department of the Navy.
Last fall, that proposal was scrapped amid resistance from senior Pentagon officials, including Defense Secretary Jim Mattis, Air Force Secretary Heather Wilson and Air Force Chief of Staff Gen. David Goldfein, who said it would create unnecessary costs and bureaucracy.
"I oppose the creation of a new military service and additional organizational layers at a time when we are focused on reducing overhead and integrating joint warfighting functions," Mattis said in October in a memo to Sen. John McCain, chairman of the Senate Armed Services Committee.
Retired Air Force Lt. Gen. Dave Deptula, dean of the Air Force Association-founded Mitchell Institute for Aerospace Studies, described the decision to create a Space Force as "another example of ready, fire, aim," in a Monday news briefing.
The announcement was made at a meeting of the National Space Council, at which Trump signed a new space policy directive aimed at reducing debris in Earth's orbit. The policy sets up new guidelines for satellite design and operation, as well as tracking the growing amount of clutter in space.
But, citing the number of regulations his administration has dismantled since he took office, Trump warned the space council, "Don't get too carried away."
The president also reasserted plans to land astronauts on the moon again and, eventually, Mars. But his administration has provided few specifics about the architecture of its moon program or a timeline for returning to the lunar surface.
The Washington Post's Aaron Gregg contributed to this report.
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