Private Manning is accused of using CDs to download thousands of files from a secure military network that he had access to while stationed in Iraq and then giving them over to Julian Assange, the head of WikiLeaks, who published them online and shared them with a number of media organisations including the Daily Telegraph.
Among the files, considered the largest intelligence leak in American history, were secret assessments of foreign leaders by US diplomats, logs from the wars in Iraq and Afghanistan and video footage showing an airstrike in Iraq in which Army helicopters killed two journalists.
Private Manning was arrested after a computer hacker he spoke to online approached the FBI and showed them chat logs where the soldier described how releasing the data could lead to "worldwide discussion, debates and reforms".
For almost a year he was held in virtual solitary confinement at a Marine base in Virginia, where his treatment was protested by Amnesty International and the UN chief investigator on torture.
His civilian defence lawyer, David Coomb, is expected to argue that Private Manning was emotionally unstable at the time of his deployment to Iraq and should never have been given access to the secure network.
Earlier this year Mr Coomb submitted a request for 48 witnesses to appear at the hearing, including President Barack Obama and Secretary of State Hillary Clinton, of which only 10 military and law enforcement figures were approved.
The defence hoped to question Mrs Clinton about the impact of the leak on American foreign policy, while arguing that the President had jeopardised Private Manning's chance of a fair trial by saying he "broke the law" at a fundraiser earlier this year.
It is not clear whether Private Manning himself will speak at the hearing, which could last up to a week.