Washington — President Obama will address the nation tonight from the National Defense University, which sits on the southwest lip of the capital on the grounds of Fort McNair. The small auditorium is already packed, the audience mostly military personnel and members of the press. The stage setting is simple: eight American flags, the podium, and a navy-blue, draping curtain. The crowd is chattering, of course, about Libya. But there is a real buzz in the room. These young men with crew cuts, and young women, are stoked about seeing their commander-in-chief. For the moment, Adm. Mike Mullen, the chairman of the Joint Chiefs of Staff, is making the rounds, shaking hands and holding court up front.