Shots fired. The Marine Corps’ seven Marine Expeditionary Units are designed to deploy at a moment’s notice for combat or emergency response, but it seems two of the MEUs have been waging a war a little closer to home. The 15th MEU, out of Camp Pendleton, Calif., has had an official account on Twitter for over a year, tweeting a regular stream of Marine Corps news and photos. But when its neighboring 13th MEU joined Twitter in July, things started heating up. On Aug. 23, the 13th MEU account issued a friendly shout-out to all the MEUs on Twitter. [HTML1]…