FORT MEADE, Md. — A military prosecutor portrayed Pfc. Bradley Manning on Thursday as an “anarchist” who, seeking to “make a splash,” betrayed the United States’ trust when he leaked vast archives of secret documents to WikiLeaks, lifting a veil on American diplomatic and military activities.

As closing arguments began in the high profile court-martial trial, the prosecutor, Maj. Ashden Fein, focused squarely on the most contentious charge that Private Manning is facing: that by giving the information to WikiLeaks for publication on a Web site that the world could see, he is guilty of “aiding the enemy.”

That charge has never been brought in a leak case, and the theory behind it could establish a precedent with implications for investigative journalism in the Internet era. But Major Fein said it was justified in Private Manning’s case. Prosecutors are seeking a life sentence.

“Pfc. Manning was not a humanist; he was a hacker,” Major Fein said, adding: “He was not a whistle-blower. He was a traitor, a traitor who understood the value of compromised information in the hands of the enemy and took deliberate steps to ensure that they, along with the world, received it.”