Manning, who faces a possible 154-year jail sentence, has offered to plead guilty to several offences but he denies prosecutors' most serious charge -- that he knowingly aided the enemy, chiefly al Qaeda and Osama Bin Laden.
The trial follows an exhaustive series of preliminary hearings that outlined the government's case against Manning, 25, over leaks of diplomatic cables and war logs to the anti-secrecy website that caused huge embarrassment to the United States and its allies.
The soldier's supporters argue that his actions shone a light in the darkest corners of the US-led wars in Iraq and Afghanistan, as well as exposing the reasoning behind American foreign policy decisions.
His legal defense team has successfully argued that he was subjected to unduly harsh detention methods from US military personnel and, consequently, he will receive a 112-day reduction of any eventual jail sentence.
However, the US Army private recently admitted he was the source of the WikiLeaks disclosures and he appears certain to be found guilty.
By offering to plead guilty to 10 offenses, including breaches of military discipline and good conduct, he could be sentenced to 20 years in jail. But if he is convicted of the more serious charges he faces life imprisonment.
While the case has served as a cause celebre for civil liberties advocates in the United States, the government argues that Manning's actions helped the nation's enemies in a political era defined by the threat posed by al Qaeda.
His courts-martial, which is expected to last 12 weeks, is taking place at Fort Meade military base in Maryland.
Although dozens of reporters are covering the trial, some evidence will be given behind closed doors for national security reasons.
The government's refusal to publish legal documents during the pre-trial proceedings against Manning was heavily criticized by media outlets, leading to allegations of unnecessary official secrecy.