Who has control over censorship during war time?

I am utterly confused. I need some solid evidence. Who, if anyone, has control over the press and censorship during wartime, specifically the Vietnam war. At first I thought there had to be a declaration of war for censorship to take place, bu then i found that wasn't necessarily true, like the Korean war, the Persian Gulf war etc.. Then I thought about the secretary of defense and wondered whether Robert McNamara played a part in censorship during Vietnam. I could really use some help.