FORT MEADE, Md. — The court-martial of Pfc. Bradley Manning, whose secret release of a vast archive of military and diplomatic materials put WikiLeaks into an international spotlight, opened here Monday with dueling portrayals of a traitor who endangered the lives of his fellow soldiers and of a principled protester motivated by a desire to help society who carefully selected which documents to release.

The contrast between the government’s description of Private Manning and his lawyer’s underscored the oddity at the heart of the trial, which is expected to last as long as 12 weeks: There is no doubt that he did most of what he is accused of doing, and the crucial issue is how those actions should be understood.

In February, Private Manning pleaded guilty to nine lesser versions of the charges he is facing — and one full one — while confessing in detail to releasing the trove of documents for which he could be sentenced to up to 20 years.

But his plea was not part of any deal and prosecutors are going to trial because they hope to convict him, based on essentially the same facts, of 20 more serious offenses — including espionage and aiding the enemy — that could result in a life sentence.