Anti-war protesters convicted of aggravated trespass at various military bases were denied justice at their trials, two High Court judges heard.
Some protesters occupied tanks at a military port in Southampton
Maurice Mendelson QC accuses the three courts that tried their original cases of wrongly blocking this defence.
The 16 activists should not have been found guilty because they were trying to stop an "illegal" war, he said.
The protests over the invasion of Iraq took place at bases in Hampshire, Northamptonshire and Gloucestershire.
'Crime against peace'
Fourteen of the protesters were Greenpeace activists who were convicted of aggravated trespass (disrupting lawful activity) after they occupied tanks at Marchwood military base in Southampton in February 2003.
Also being heard is the case of Lindis Percy, joint co-ordinator of the Campaign for the Accountability of American Bases (CAAB), who was convicted of aggravated trespass after hanging an anti-war flag at the US Air Force base at Croughton, Northamptonshire, in March 2003.
The final case is that of Valerie Swain, convicted of aggravated trespass and criminal damage after cutting the fence at RAF Fairford, Gloucestershire, entering the base and causing disruption.
The protesters say they were acting to stop a "crime against peace" being committed.
'Illegal war' defence
At their original trials, each raised the same defence, that the Government's support of the US in the Iraq war involved a war crime under the International Criminal Courts Act (ICCA) - and preparations for the war at the bases were therefore unlawful.
At the High Court on Tuesday, Mr Mendelson told Lord Justice Waller and Mr Justice Jack that the trial judges had erred in law in all three cases by not allowing their "illegal war" defence because it related to decisions concerning defence and foreign policy.
Mr Mendelson argued that the court was also wrong to refuse the Marchwood protesters' request for access to government documents to support their anti-war argument.
Lord Justice Waller told the QC that his burden was to show that there was an arguable case that personnel at the bases had been acting unlawfully.
However, Mr Mendelson told the court that it was for the Crown to prove the opposite - that personnel were not aiding and abetting an illegal war.
The appeal is expected to continue for one more day.