Manning has admitted to sending troves of material to the anti-secrecy website WikiLeaks and pleaded guilty to charges that would send him to prison for up to 20 years. The U.S. military and the Obama administration weren't satisfied, though, and pursued a charge of aiding the enemy, which carries a potential life sentence.
The trial on that most serious charge and 20 other offences begins Monday for the 25-year-old former intelligence analyst from Oklahoma. It's the most high-profile case for an administration that has come under criticism for its crackdown on leakers. The six prosecutions since President Barack Obama took office is more than in all other presidencies combined.
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Manning chose to have his court-martial heard by a judge instead of a jury. It is expected to run all summer.
In February, Manning told military judge Army Col. Denise Lind that he leaked the material to expose the American military's "bloodlust" and disregard for human life in Iraq and Afghanistan. He said he did not believe the information would harm the U.S. and he wanted to start a debate on the role of the military and foreign policy.
The judge accepted his guilty plea to reduced charges for about half of the alleged offences, but prosecutors did not and moved forward with a court-martial on charges including violations of the Espionage Act and the Computer Fraud and Abuse Act.
Manning's supporters hail him as a whistleblowing hero and a political prisoner. Others view him as a traitor.
U.S. officials have said the more than 700,000 Iraq and Afghanistan battlefield reports and State Department cables sent to WikiLeaks endangered lives and national security.
The material WikiLeaks began publishing in 2010 documented complaints of Iraqi detainee abuses; a U.S. tally of civilian deaths in Iraq; and America's weak support for the government of Tunisia — a disclosure Manning supporters said encouraged the popular uprising that ousted the Tunisian president in 2011 and helped trigger the Middle Eastern pro-democracy uprisings known as the Arab Spring.
Last month, the government agreed to accept Manning's guilty plea for one lesser version of one count, involving a single diplomatic cable summarizing U.S. embassy discussions with Icelandic officials about the country's financial troubles.
Manning also acknowledged sending WikiLeaks unclassified video of a 2007 U.S. Apache helicopter attack that killed civilians, including a Reuters photographer. An internal military investigation concluded the troops reasonably mistook the camera equipment for weapons; WikiLeaks dubbed the video "Collateral Murder."
The release of the cables and video embarrassed the U.S. and its allies. The Obama administration has said it threatened valuable military and diplomatic sources and strained America's relations with other governments, but the specific amount of damage hasn't been publicly revealed and probably won't be during the trial.
Lind ruled the extent of any damage is irrelevant. Defence attorney David Coombs contends it was minimal.
Much of the evidence is classified, which means large portions of the trial are likely to be closed to reporters and the public. The judge tested alternatives to closing the courtroom, such as using code words and unclassified summaries, but Lind said it didn't work.