The US Department of Defence confirmed that computer systems controlled by the Defence Information Systems Agency (DISA) had been hacked, exposing the personal data of about 200,000 people.
The agency oversees military communications including calls for US President Donald Trump.
The data exposed included names and social security numbers.
A spokesperson for the Department of Defence would not say whether the organisation knew who was responsible for this attack - but told the BBC the department was constantly under threat.
"DoD networks are under attack daily and the department maintains an active posture to thwart those attacks," the spokesperson said.
The agency said it has begun notifying people who had been affected but that there was "no evidence" their information has been misused.
The data breach occurred over the summer of 2019 and letters to possible victims began going out this month.
where the hell is the U.S. cybercommand???