The attack appeared to be the work of the Syrian Electronic Army, which also recently targeted the New York Times' website and Twitter.
The hackers posted a message and images on the website www.marines.com, signing it "delivered by SEA," a reference to the Syrian Electronic Army. A Defense Department spokesman said the site, on commercial network rather than the Defense Department network, had been restored after an outage of a few hours.
The seven-sentence "Message to the United States Marine Corps," said the Syrian Army "should be your ally, not your enemy" against "a vile common enemy" of terrorism.
"Refuse your orders," said the message which included six photos of people in military-style uniforms, their faces obscured and holding hand-written messages, such as "I will not fight for Al Qaeda in Syria."
The White House says the Syrian government used chemical weapons to kill 1,400 people, many of them children, on August 21. It has asked Congress to approve punitive action against Syria.
© Thomson Reuters 2013