Anti-war protesters are at the centre of a landmark case at the House of Lords that could test the legality of the Iraq war.
Some protesters occupied tanks at a military port in Southampton
Five law lords are hearing appeals by 20 people who took part in protests aimed at preventing military action.
Fourteen say they should not have been convicted of aggravated trespass near Southampton docks because they were trying to stop an "illegal war".
The same argument is being used by five others who entered RAF Fairford.
'Shock and awe'
Fourteen of the group, known as the Marchwood 14, are Greenpeace volunteers who took part in a week of action near Southampton docks, ahead of the war in 2003.
They attempted to stop or delay military hardware leaving the country for Iraq so that the all-important "weather window" for the invasion would be missed.
Another protester was convicted of aggravated burglary after she entered RAF Fairford in Gloucestershire.
The other five are alleged to have tried to immobilise American B52 bombers at Fairford which were later involved in "shock and awe" attacks on Baghdad.
They are due to stand trial separately at Bristol Crown Court later this year and their defence will depend on the House of Lords hearing.
In earlier hearings, judges have ruled that the courts cannot deal with the legality of the war, which was considered to be a political matter.
The Court of Appeal said that acting to prevent the commission of a crime could be considered as a defence, but added that in reality in cases like this "they were protests and were not attempts to prevent crimes".
'Crime of aggression'
Rabinder Singh QC, defending 15 of the group, told the Law Lords, headed by Lord Bingham, that the protesters were being wrongly deprived of a defence under which the legality of the war would have been questioned and which could have resulted in their acquittals.
Greenpeace executive director Stephen Tindale said before the hearing, which is due to end next Monday: "This case is crucial as it could help ensure that the UK is never again dragged into an illegal pre-emptive war, against UN wishes and based on dodgy intelligence.
"If we are to have a society where governments as well as individuals can be held accountable for their actions and the international rules of law are upheld, then it's crucial that the right to this defence of preventing a crime of aggression is upheld."