Have been meaning to write about the work of the "Syrian Electronic Army".
As mentioned in the article above, they have been very active. The latest target is a "military" one, compared to the previous attacks on news websites.
Marines.com is not the main USMC website. It's more of a promotional website for the Marines. So it is likely that no serious infiltration or loss of information has occurred.
The "attack" is more likely a hack on the DNS, to direct users who type www.marines.com to another website, instead of the real Marines.com website.
It is more an asymmetric attack. But it also brings to reality the many articles written about cyber warfare. Some have been dismissive, but it seems we no longer can ignore this threat.