The website has general information about the Army, press releases and Army-generated news stories. It contains no classified information or private personal data of any personnel, the Army said.
"Today an element of the Army.mil service provider's content was compromised," spokesman Brigadier General Malcolm Frost said.
The Army took down the website temporarily after the hack was discovered to ensure there was no breach of Army data.
The hackers demanded the U.S. stop training rebel fighters inside Syria. A message posted at the site said, "Your commanders admit they are training the people they have sent you to die fighting."
A group called the Electronic Syrian Army claimed responsibility for the hack, according to a report. The group was behind other recent hacks of other websites including The New York Times and Twitter.
©2015 Deutsche Presse-Agentur GmbH (Hamburg, Germany) Distributed by Tribune Content Agency, LLC.
