President Trump signed a directive Tuesday setting in motion a scaled-down version of his Space Force to be housed within the Air Force, though it may one day become a co-equal branch of the military.

Facing skepticism both at the Pentagon and on Capitol Hill to the need for the first separate new service for the U.S. military since 1947, Mr. Trump has argued the new force is needed to counter growing space-related threats from China, Russia, Iran and others, who are developing new weapons to interfere with satellites.

“Whether we like it or not, that’s where we’re going. It’s space. That’s the next step, and we have to be prepared,” Mr. Trump said in an Oval Office signing ceremony with Vice President Mike Pence, acting Defense Secretary Patrick M. Shanahan and other top officials.

Space Policy Directive 4 orders the Department of Defense to devise a budget for the new force that can be included in Mr. Trump’s fiscal 2020 request to Congress. The force would have a new civilian undersecretary position for space, to be appointed by the president and approved by the Senate.

Mr. Trump last year suggested the Space Force would be a sixth branch fully on a par with the Navy, Army or Air Force. Yet the proposal ran into opposition on Capitol Hill, where some lawmakers criticized the change as expensive and unnecessary. For now, the new force will be run within the broader Air Force, akin to the Marine Corps’ status under the Navy.

Defense Department leaders expect to formally unveil legislative language at the Pentagon next week, an Air Force official told The Washington Times. He said a budget has been prepared but declined to comment on specific figures. White House officials told Stars and Stripes that the original outlay would be just $100 million, far below the Pentagon’s initial estimate of $13 billion.

The move allows Mr. Trump to put his stamp on a Pentagon reorganizing to fight emerging rivals such as China and Russia. But the initial proposal is a significant comedown from the White House’s original concept floated last year.

“We didn’t see a way to go there in one step,” one White House official told Politico. “The thought was to leverage the facilities and functions already within the Air Force, since that’s where the bulk of space capabilities really are.”

Gen. David Goldfein, chief of staff of the Air Force, on Tuesday said roughly “80 or 90 percent” of space-related military personnel work under the Air Force. “We have been in a robust debate, as you can imagine,” on how the new force will operate, he said at a forum organized by the Brookings Institution.

Mr. Trump’s directive is likely to face push back from lawmakers who see the added expense as unnecessary, and it is not guaranteed to pass in the Democrat-controlled House.

“Congress will almost certainly take their time evaluating the proposal, possibly making significant changes to the legislation before voting on it,” said Kaitlyn Johnson, an associate director at the Center for Strategic and International Studies who specializes in military space systems.

Still, the White House will be able to go forward with creating a new combatant command for space, as the move does not require congressional approval until Mr. Trump decides to create a separate department.

Although expected to be smaller than the existing military services, the Space Force could prove a jobs-generating plum for the state or states that host it. Florida Gov. Ron DeSantis, a Republican and key Trump ally, said his state would be the “logical choice” to house the command, given its experience with the space industry.

Texas Rep. Mac Thornberry, ranking Republican of the House Armed Services Committee, noted the Space Force idea “has a record of attracting bipartisan support in Congress.”

“This is an important next step towards real reform of national security space where we face real threats posed by Russia and China,” he said.

• Carlo Munoz contributed to this story.

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