Fein said Manning betrayed his country's trust and spilled classified information, knowing the material would be seen by the terrorist group al-Qaida.
"WikiLeaks was merely the platform which Pfc. Manning used to ensure all the information was available for the world, including enemies of the United States," Fein said.
Manning, 25, is charged with 21 offenses, but the most serious is aiding the enemy, which carries a possible sentence of up to life in prison. A native of Crescent, Okla., Manning has acknowledged giving WikiLeaks some 700,000 battlefield reports, diplomatic cables and videos. But he says he didn't believe the information would harm troops in Afghanistan and Iraq or threaten national security.
A military judge, not a jury, is hearing the case at Manning's request. Army Col. Denise Lind will deliberate after closing arguments.
The verdict and any sentence will be reviewed, and could be reduced, by the commander of the Military District of Washington, currently Maj. Gen. Jeffery S. Buchanan.