British Prime Minister Gordon Brown in June called for an inquiry to examine the eight-year period beginning with the push for war in 2001 to July 2009 when British forces ended their mission there.
John Chilcot, appointed as the chair of the inquiry into the circumstances surrounding the invasion, heard the first testimony for the inquiry Nov. 24.
Maj. Gen. David Wilson, who served as the British adviser to U.S. Central Command under Prime Minister Tony Blair, said U.S. commanders revealed their plans for possible military action in Iraq in June 2002, the BBC reports.
"This is when, not just the British but the Australians, were made privy to planning that had gone to that point by the U.S.," he said.
In August U.S. military planners asked their British and Australian counterparts for their assessment of the plan. Wilson responded, "Unless political and legal issues are resolved it would be difficult for the U.K. to deliver even basic support."
The inquiry will tackle issues regarding the legality of the war when Blair is expected to testify in early 2010.
Families of British soldiers killed in Iraq are expected to confront the former premier during the examination.