Standing in front of the Space Shuttle Discovery, Vice President Pence‬ delivers remarks before convening the first meeting of the reestablished National Space Council at the National Air and Space Museum’s Steven F. Udvar-Hazy Center in Chantilly, Va., last year. (Jonathan Ernst/Reuters)

The Pentagon is moving toward fulfilling President Trump’s request to establish a Space Force, in what would be the first new branch of the military in more than 70 years.

In a speech at the Pentagon on Thursday at 11:15 a.m., Vice President Pence is scheduled to lay out the administration’s plan. The Pentagon is also expected to release a congressionally mandated report on the plan.

But the calls for a separate military branch have been met with strong reluctance in some parts of the Pentagon amid concerns that it doesn’t need the burdens of a new bureaucracy. The move could significantly reorganize the military and potentially strip the Air Force of some of its key responsibilities.

Last year, Defense Secretary Jim Mattis said in a memo to Sen. John McCain (R-Ariz.), chairman of the Senate Armed Services Committee, that he opposed “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.”

On Tuesday, however, Mattis said that military leaders “are in complete alignment with the president’s concern about protecting our assets in space to contribute to our security to our economy and we’re going to have to address it as other countries show a capability to attack those assets.”

Congressional approval would be needed to establish an entirely new military branch. A Space Force, dedicated to space the way the Navy is to the sea, would be the first new military service since the Air Force was created in 1947. But Mattis said that creating a new combatant command for space, such as one that governs the Pentagon’s Special Operations, “is certainly one thing that we can establish.”

He added that the plan for how best to implement it was still ongoing: “I don’t have all the final answers yet. We’re still putting it together.”

Defense One, a news outlet focused on the military, recently reported that the combatant command would be led by a four-star general and that there would also be an agency focused on buying satellites, citing a draft copy of a report due to Congress soon.

For years, the Pentagon has been warning about how space has become a contested domain of war — just like the land, air and sea. And it has become increasingly concerned that its assets in space are vulnerable to attack.

Military leaders have said repeatedly that modern warfare depends on space. The Pentagon and intelligence community has a host of sensitive satellites that perform all sorts of vital national security tasks, such as missile warning, precision-guided munitions, military communications and intelligence. But how best to protect those assets — and deter other potential adversaries from attacking them — has touched off an intense debate in Washington.

Last year, some members of Congress proposed creating a “Space Corps” inside the Air Force, similar to how the Marine Corps is part of the Navy Department. Senior Pentagon officials opposed the measure, and the plan was shelved.

Trump’s Space Force plan would go even further, creating a new branch with both a new member of the Joint Chiefs of Staff and a political appointee serving as a service secretary.

During a speech in June, Trump directed the Pentagon “to immediately begin the process necessary to establish a Space Force as the sixth branch of the Armed Forces.” And he said that “we are going to have the Air Force, and we are going to have the Space Force — separate but equal.”

Military officials are concerned about threats posed by China and Russia in particular. In 2007, China destroyed one of its dead weather satellites with a missile. Then a few years later, it fired another missile, this time into the deeper orbit where the Pentagon parks its most sensitive satellites.

That made it clear to the Pentagon that space, long seen as a peaceful domain, was a place where wars could be fought and won.

“There’s not a mission today that we do in the military that doesn’t in some way depend upon space,” Air Force Sec. Heather Wilson said at an event last month at The Washington Post. “But we built that architecture in space at a time when it was benign. We built the glass houses before the invention of stones. So now, we have to adjust and make sure that we can defend what we do in space and deter anyone from challenging us there.”

But the plan to create an entirely new branch of the military has rankled some in the Pentagon, who say it is unnecessary and that the Air Force has for years looked after the nation’s interests in orbit.

This year, Gen. John Hyten, the commander of the U.S. Strategic Command, said that “rather than going through the difficulty of new organizational constructs, I believe the leadership we have now can execute the missions that we need to.”

Since then, the White House has continued to press for a Space Force, giving the plan momentum and the weight of the executive branch.

“We have the direction from the president, and we’re underway,” Mattis said Tuesday.