The US private accused of being behind the release of a raft of WikiLeaks documents was forced to stay in his cell for up to 23 hours a day and sleep naked, a pretrial hearing into the charges heard on Tuesday.
Bradley Manning's lawyers argue that he was illegally punished by being locked alone in a small cell for nine months.
Judges can dismiss all charges if pretrial punishment is too harsh but it normally results in a reduced sentence.
Mr Manning's supporters packed the courtroom, many wearing t-shirts bearing the slogan "truth." They say his disclosures exposed a litany of war crimes and aggressive US foreign policy, helping spark mass pro-democracy demonstrations in the Middle East.
But the US claims the leaks endangered national security and put lives at risk.
Much of the first day of proceedings concentrated on the harsh conditions Mr Manning faced in the brig.
He was considered to be at risk of injuring himself or others and was placed in maximum custody despite the recommendation of mental-health workers who examined him.
Quantico marine corps base - where Mr Manning was held - installation commander Daniel Choike claimed the private had "danced erratically" and licked the bars.
Manning lawyer David Coombs put that down to attempts at exercising during his long hours of confinement and said Mr Manning sleepwalked.
Mr Manning faces a possible life sentence if he's found guilty of the most serious charge against him - aiding the enemy.
Foreign Minister Alistair Burt's admission that the Cameron government has "supported" a survey of attitudes to US drone strikes in Pakistan's tribal areas amounts to a tacit admission of British involvement.