The US Army took down its homepage on Monday following an attack by the Syrian Electronic Army. Other government sites, including US Strategic Command, were also down. Posted messages included "Your commanders admit they are training the people they have sent you to die fighting" and "Hacked by the Syrian Electronic Army" (according to the SEA Twitter feed).
At Ars Technica, Sean Gallagher reports that the internet defacement appeared to be accomplished through a third-party vendor's systems.
Based on screenshots published in the Syrian Electronic Army's Twitter account, it appears the attack gained access to the webpage through the Limelight Networks content delivery network. A screenshot shows a Limelight control panel for the account belonging to the US Army Office of Public Affairs. [Update: A spokesperson from Limelight contacted by Ars said, "We take security concerns extremely seriously and, in an abundance of caution, we are conducting a full investigation. At this point we have no reason to believe any customer data has been compromised."]