WASHINGTON — President Trump on Thursday authorized the creation of the United States Space Command, citing the need for a centralized unit to protect American interests in what he called “the next war-fighting domain.” He described the command as a precursor to the Space Force, a sixth branch of the military that he has promised to supporters at his rallies and that he wants Congress to create.

“The Space Force will organize, train and equip warriors to support Spacecom’s mission,” Mr. Trump said during a Rose Garden ceremony authorizing the command, which he said would assert American dominance in space “because we know the best way to prevent conflict is to prepare for victory.”

The idea of establishing a unified command to coordinate the military’s role in space is not new — President Ronald Reagan created something similar, but it was disbanded after the Sept. 11, 2001, attacks, when fighting terrorism became the Pentagon’s priority.

But 17 years later, there is little doubt at the Pentagon about the need to protect American interests against global competitors like Russia and China in the event they target space-based American technology, especially the satellites that help guide aircraft carriers in the North Arabian Sea and fighter jets over the Persian Gulf and that enable intelligence officers to gather information on adversaries.