Fort Meade - US military prosecutors demanded on Monday that Private Bradley Manning spend at least 60 years in jail for handing a vast trove of secret files to anti-secrecy site WikiLeaks.
Captain Joe Morrow urged trial judge Colonel Denise Lind to impose a tough sentence to "send a message to any soldier contemplating stealing classified information".
The 25-year-old former army intelligence analyst has been convicted on a raft of espionage and theft charges that could see him jailed for more than a century.
But Lind is conducting a sentencing at Fort Meade, a military base just outside Washington, to decide how long the young man, who has apologised, should serve.
Morrow dismissed the defense argument that Manning was a naive and troubled soldier who believed he was doing good by exposing what he saw as abuses in America's conduct of the wars in Iraq and Afghanistan.
Instead, the prosecutor branded the leaks "destructive" and said Manning was a "determined insider who exploited an imperfect system".