They are said to include 250,000 diplomatic cables and 500,000 battlefield reports from Afghanistan and Iraq in 2009 and 2010. The Welsh born army soldier has been charged with 22 offences and has pleaded guilty to ten, not including the most serious of aiding the enemy.
If found guilty Private Manning faces a maximum term of 154 years in jail.
The judge, Colonel Denise Lind, said last month she would close parts of the trial to the public to protect classified material.
Lind began the trial on Monday by asking Manning a number of procedural questions, including whether he was willing to have the case decided by a judge rather than a jury and whether he was satisfied with his defence team.
“Yes, your honour,” replied Manning, who was arrested in May 2010 while serving in Iraq.
The prosecution said Bradley Manning dumped classified documents on the internet and into the enemy’s hands.
Captain Joe Morrow made the comments on Monday during his opening statement.
Morrow says Manning’s case is about what happens when arrogance meets access to sensitive information.
Prosecutors are trying to prove Manning aided the enemy.
Manning says he released the material to the anti-secrecy website WikiLeaks to expose the American military’s “bloodlust” and disregard for human life in Iraq and Afghanistan. He says he did not believe the information would harm the US.
Manning’s supporters hail him as a whistleblowing hero and a political prisoner. Others view him as a traitor.