Which, to my mind, does not justify those documents being classified. Which seems to be the way things happen anymore. If a politician does something wrong, or stupid, or embarrassing, it gets classified so that he or she won't have to face responsibility for it.
I'm not all that familiar with the kinds of things Manning revealed, but exposing documents which show that, for example, some high ranking official ordered an illegal drone strike on a non-military target resulting in civilian casualties, I wouldn't consider revealing those documents to be treason. The person who authorized the strike, then tried to cover it up, is the one who has committed treason, in that he involved his country in an illegal act. THAT person should be held accountable, not the person who revealed his actions.