"Your honor, he was not a whistleblower, he was traitor."
As an intelligence analyst in Iraq, Manning had pledged under oath to safeguard sensitive information held by the government, but he "abused and destroyed this trust," Fein said.
The 25-year-old Manning has already admitted to leaking hundreds of thousands of classified diplomatic cables and battlefield intelligence reports to the anti-secrecy website WikiLeaks.
But he has denied other charges against him, including the most serious count that he knowingly helped Al-Qaeda.
That charge carries a possible life sentence, and civil liberties groups have warned that a guilty verdict could have a chilling effect on government whistleblowers and journalistic inquiry.
Displaying a photo of Manning smiling and looking "gleeful" -- allegedly after he began his document dump to WikiLeaks -- Fein said evidence in the court-martial showed Manning had wanted to wreak havoc.
Manning "wasn't interested in oaths," Fein said. "He was interested in making a name for himself."
Working in military intelligence in Iraq, Manning had been trained to know that "terrorists" use the Internet to gather information for attacks against the United States, he said.
Manning was aware that WikiLeaks had been identified in three military intelligence reports as a possible threat to national security, as the site sought to expose classified material, according to Fein.
The prosecution depicted Manning as agent for WikiLeaks after having corresponded with founder Julian Assange and others in the organization.
From November to December 2009, Manning was "searching for topics related to one mission -- finding and disclosing what WikiLeaks wanted," Fein said.
The material Manning leaked included a cockpit video of a US Apache helicopter attack in Iraq that killed 12 people, including two Reuters employees.
Fein said Manning was irresponsible in releasing the video and that he did it because he thought it would be "cool" to pass the footage to a "bunch of anti-government activists and information anarchists."
In an online chat, Manning described the impact of his document dump as a "beautiful and horrifying thing," the prosecutor said.
"These are not the words of a humanist, but the words of an anarchist."
The defense is due to offer its closing argument on Friday. Then the judge, Colonel Denise Lind, will deliver a verdict possibly as soon as this weekend, three years after Manning was arrested in Iraq.
Born in Oklahoma to an American father and a Welsh mother, the slight, bespectacled Manning has said he wanted to shed light on US foreign policy abuses.
Manning says he believed the reports he saw in his job "needed to be shared with the world" and that doing so "would help document the true cost of the wars in Iraq and Afghanistan."
He has pleaded guilty to 10 lesser chargers of federal espionage, computer fraud and wrongful storage of classified information, which could carry a sentence of up to 20 years in prison.
Prosecutors are trying to prove 12 additional counts, including theft of US property, exceeding authorized access on a government computer and the "aiding the enemy" charge.
Even if he is acquitted of aiding the enemy, Manning faces a possible sentence of 154 years behind bars.
The court-martial has taken on added importance in the wake of revelations from another young man working with classified information, Edward Snowden.
The former IT contractor at the National Security Agency recently blew the lid on US surveillance of phone records and Internet traffic.
The outcome of Manning's trial could have implications for the case against Snowden, if he is ever extradited to the United States after having fled to Moscow via Hong Kong.