Military commanders are expected to tell the inquiry into the Iraq war, which opens on Tuesday, that the invasion was ill-conceived and that preparations were sabotaged by Tony Blair's government's attempts to mislead the public.

They were so shocked by the lack of preparation for the aftermath of the invasion that they believe members of the British and US governments at the time could be prosecuted for war crimes by breaching the duty outlined in the Geneva convention to safeguard civilians in a conflict, the Guardian has been told.

The lengths the Blair government took to conceal the invasion plan and the extent of military commanders' anger at what they call the government's "appalling" failures emerged as Sir John Chilcot, the inquiry's chairman, promised to produce a "full and insightful" account of how Britain was drawn into the conflict.

Blair had in effect promised George Bush that he would join the US-led invasion when, as late as July 2002, he was denying to MPs that preparations were being made for military action. The leaked documents reveal that "from March 2002 or May at the latest there was a significant possibility of a large-scale British operation".

Documents leaked in 2005 show that, almost a year before the invasion, Blair was privately preparing to commit Britain to war and topple Saddam Hussein, despite warnings from his closest advisers that it was unjustified. They also show how Blair was planning to justify regime change as an objective, despite warnings from Lord Goldsmith, the attorney general, that the "desire for regime change was not a legal base for military action. ...