President Trump has launched America's first new military service in 70 years - the US Space Force.
Mr Trump said "space is the world's new war-fighting domain" as he signed off the $1.4 trillion (£1.08 trillion) National Defence Authorisation Act.
Space Force is part of a package that also provides money for a US-Mexico border wall.
During a ceremony at Joint Base Andrews in Maryland, the president said: "Among grave threats to our national security, American superiority in space is absolutely vital.
"And we're leading, but we're not leading by enough, and very shortly we'll be leading by a lot."
Space Force will protect US assets, including the hundreds of satellites used for communication and surveillance.
It comes as China and Russia embark on a major programme to develop technologies that could allow them to disrupt or destroy American and allied satellites during a crisis or conflict.
A report for the Pentagon in February said America faced "serious and growing challenges to its freedom to operate in space".
Space Force is the first new military service since the air force was separated from the army in 1947.
It will provide forces to US Space Command, an organisation re-established this year to oversee the military's space operations.
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It will initially be made up of around 200 people and in its first year have a $40m (£30m) budget.
While the service is tiny compared with the army - which boasts around 480,000 soldiers and a budget of $181bn (£140bn) - Vice President Mike Pence has said Space Force is "the next great chapter in the history of our armed forces".
Kaitlyn Johnson, a space policy expert at the Centre for Strategic and International Studies, said the new force was as an important move but doubts it will be as momentous as Trump officials suggest.
She said opposition to making Space Force a separate branch of the military means it could be curtailed or scrapped if a Democrat is elected president in November.
She added: "Just because it's written into law doesn't mean it can't be unwritten
"Because of the politics that have started to surround the Space Force, I worry that that could damage its impact before it even has time to sort itself out."