“I am hereby directing the Department of Defense and Pentagon to immediately begin the process necessary to establish a Space Force as the sixth branch of the armed forces,” President Trump said in remarks on Monday.

The president made this request during a meeting at the White House with the National Space Council, saying “Our destiny beyond the Earth is not only a matter of national identity but a matter of national security.”

Fortune contacted the Department of Defense for comment but has not heard back.

BREAKING: Space Force is real. "We must have American dominance in space. Very importantly, I am hereby directing the Department of Defense and Pentagon to immediately begin the process necessary to establish a Space Force as the Sixth Branch of the Armed Forces," Trump says. pic.twitter.com/8WRhqYW5Wj

— Christina Wilkie (@christinawilkie) June 18, 2018

This is not the first time that Trump has mentioned a specific military branch or “space force” dedicated to space. In March, in a speech to a group of marines, he said the “space force” started as a joke, but then became a serious idea.

“Space is a war-fighting domain, just like the land, air, and sea,” Trump said in March. “We may even have a Space Force, develop another one, Space Force. We have the Air Force, we’ll have the Space Force.”

Secretary of Defense James Mattis has previously opposed the creation of a space-focused branch of the military on bureaucratic grounds. Last year, he wrote to the Senate Committee on Armed Services regarding the National Defense Authorization Act for fiscal year 2018, and among his areas of concern included what Congress was calling “Space Corps.”

“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 efforts,” he wrote.

At the end of 2017, the Space Corps provision was taken out of the House version of the NDAA where it originated.

The Outer Space Treaty of 1967, which lays out space law, specifically states that the moon and celestial can only be used for “peaceful purposes” and prohibits countries from placing “nuclear weapons or other weapons of mass destruction in orbit or on celestial bodies or station them in outer space in any other manner.”

However, because the treaty does not specify conventional weapons, countries such as the U.S., China and Russia are looking into ways to expand defenses to space — such as anti-satellite missiles, etc. All three have refused to sign the U.N.’s resolution on Prevention of an Arms Race in Outer Space, according to CNN.