Tony Blair's government breached its duty to service personnel by failing to ensure the invasion of Iraq was lawful and justified, Law Lords have heard.
Fusilier Gentle died in a roadside bomb attack in June 2004
Beverley Clarke and Rose Gentle, the parents of two dead soldiers, are attempting to secure a public inquiry into British armed involvement in Iraq.
The mothers say soldiers have the right not to have their lives put at risk in illegal conflicts.
A committee of nine Law Lords is hearing their appeal.
Rabinder Singh QC, for the parents, said a duty "is owed to soldiers who are under the unique compulsory control of the state and have to obey orders".
"They have to put their lives in harm's way if necessary because their country demands it," he said.
"There is what some people call a military covenant between the state and those who are literally prepared to put their lives at risk for the sake of their country."
Trooper David Clarke, 19, of Littleworth, Staffordshire, died in a "friendly fire" incident west of Basra in March 2003.
The body of Trooper Clarke has never been found. A coroner said his death had been a "completely avoidable tragedy".
Fusilier Gordon Gentle, from Glasgow, was killed in a roadside bomb attack in Basra in June 2004. The 19-year-old was in the Royal Highland Fusiliers.
Mr Singh added: "These mothers wish me to emphasise that they have come to court with reluctance. They are proud of their sons, who died with honour serving their country."
Trooper Clarke was killed by "friendly" fire in March 2003
Mr Singh said it had become clear the overwhelming body of legal advice had been the invasion would not be lawful without a second resolution from the UN Security Council, in addition to Resolution 1441 on 8 November, 2002.
The mothers say the right to life, enshrined in Article 2 of the European Convention on Human Rights, obliges the government to take reasonable steps to ensure its service personnel do not face the risk of death except in lawful military activities.
Their claim was previously dismissed by the Court of Appeal. The legal argument before the Law Lords is expected to last three days.
The parents are demanding to know why in the space of 10 days some 13 pages of "equivocal" advice from the then Attorney General Lord Goldsmith on 7 March 2003, became one page of unequivocal advice that an invasion would be legal.
The respondents to the appeal are Prime Minister Gordon Brown, Defence Secretary Des Browne and current Attorney General Baroness Scotland.