The US soldier charged with passing classified material to whistleblowing website WikiLeaks offered guilty pleas to 10 of 22 charges against him when he appeared in court on Thursday.
But Bradley Manning denies the most serious charges against him.
A military judge said she would allow him to read a statement explaining his actions, and he told the court he wanted Americans to see "the true costs of war".
Manning would plead guilty to sending hundreds of thousands of classified documents to the anti-secrecy website WikiLeaks, in violation of military regulations but not in violation of federal espionage laws.
The judge in Fort Meade, Maryland, must decide whether to accept the guilty pleas, which could carry a sentence of 20 years in prison.
Prosecutors could still pursue a court-martial on the remaining charges, including aiding the enemy, which carries a potential life sentence.
Manning, 25, is accused of sending Iraq and Afghanistan battlefield reports, State Department diplomatic cables, other classified records and two battlefield video clips to WikiLeaks in 2009 and 2010 while working as an intelligence analyst in Baghdad.
WikiLeaks' founder Julian Assange, who is in exile inside the Ecuadorian Embassy in London, said: "Bradley Manning is America's foremost political prisoner. Today's events confirm that. Both the UN and the US military have formally found him to have been mistreated. All those involved in the persecution of Bradley Manning will find cause to reflect on their actions."
Events were held across the world last Saturday, including the UK, to mark Manning's 1,000th day in custody.