THE families of British servicemen killed in Iraq are challenging the government's refusal to hold an independent inquiry into the conflict.
But appeal judges have already told them they face "formidable hurdles" in establishing their case.
A QC for mothers Rose Gentle and Beverley Clarke told the Court of Appeal they were proud of their sons but questioned the legality of the invasion.
The government's legal team will resist a full public inquiry.
Fusilier Gordon Gentle, from Glasgow, and Trooper David Clarke, of Staffordshire, were 19 when they were killed.
Their case is being backed by Batley dad Peter Brierley, whose son Lance Cpl Shaun Brierley died in the conflict.
Ranbinder Singh QC said their deaths had left "grieving parents in whose minds there are real questions about the legality of the invasion of Iraq in March 2003".
He said their questions included the process which led to the invasion.
The families want the government to explain how 13 pages of advice from the attorney general on March 7, 2003 about the legality of the war was changed to one page by March 17 saying an invasion would be legal.
Mr Singh said: "It is reasonably arguable that there is a causal link between the assurance given by the attorney general and the deaths in question."
But QCs appearing for the prime minister, the defence secretary and the attorney general, said if that argument was accepted it would mark "a significant, constitutional shift".
Attorney General Lord Goldsmith has argued that the decision not to sanction a public inquiry was "entirely reasonable".
Last December, a High Court judge ruled they did not have a case to go to a full judicial review hearing.
But appeal judges have allowed their case to go forward.