Nine Law Lords are to rule on another legal bid to force a public inquiry into Britain's involvement in Iraq.
The mothers of two soldiers killed in Basra say the Blair government failed to ensure in advance the invasion was lawful and justified.
They are challenging a Court of Appeal ruling in December 2006 that the government was not obliged to order an independent inquiry.
Rose Gentle, who lost her son, said: "We want to know why we did go in."
Both Mrs Gentle and Beverley Clarke said the government's pledge to hold an inquiry "when the time is right" was not enough.
I would like the government to stand up and say they'll do it without the families having to put up a fight Rose Gentle
Trooper David Clarke, from Littleworth, Staffordshire, died in March 2003 in a "friendly fire" incident in the west of Basra.
Fusilier Gordon Gentle, from Glasgow, of the Royal Highland Fusiliers, died in Basra in June 2004 as a result of a roadside bomb attack.
At the centre of the families' argument is the demand for an explanation as to how 13 pages of "equivocal" advice from the former Attorney General, Lord Goldsmith, was reduced within days to one page of completely unequivocal advice that an invasion would be legal.
Mrs Gentle said she had believed the initial argument that there was an immediate threat to Britain, but she did not agree with the war after new information was unearthed.
She told GMTV: "We want to know why we did go in. They have come up with so many different excuses.
"I would like the government to stand up and say they'll do it without the families having to put up a fight."