Sean Caffrey hacked into the US Department of Defense system in 2014, the National Crime Agency (NCA) said.
He accessed and stole the ranks, usernames and email addresses of more than 800 users, as well as of about 30,000 satellite phones.
At Birmingham Crown Court Caffrey, 25, of Sutton Coldfield, admitted an offence under the Computer Misuse Act.
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The NCA said the theft took place on 15 June 2014 and Caffrey was arrested in in March 2015 after intelligence showed the hack originated from his internet connection.
Forensic examination of his computers found the stolen data on the hard drives.
Officers also found that an online messaging account linked to the attack had been opened and operated under a pseudonym using Caffrey's computers.
The Department of Defense said it cost about $628,000 to fix the damage.
Caffrey, of Lichfield Road, admitted causing a computer to perform a function to secure unauthorised access to a program or data.
He will be sentenced on 14 August.
A computer hacker has admitted stealing hundreds of user accounts from a US military communications system.