A newly declassified report says Australia's role in the 2003 invasion of Iraq was undertaken solely to enhance the alliance with the US, it's been reported.

The report, obtained by Fairfax Media under freedom of information laws, concludes that Australia's contribution was of only modest military use and, in many cases, made little sense.

The 572-page report was written between 2008 and 2011 by Dr Albert Palazzo from Defence's Directorate of Army Research and Analysis.

It concludes that then prime minister John Howard joined then US president George W. Bush in invading Iraq solely to strengthen Australia's alliance with the US.

It says that Mr Howard and then Chief of the Defence Force General Peter Cosgrove, facing domestic political pressure amid a deeply unpopular war, ensured that Australian lives were exposed to as little risk as possible.

Politically, delivering the right force was "secondary to the vital requirement of it just being there" but it led some American military officers to grumble that Australia was providing "a series of headquarters".

Mr Howard's claims of enforcing UN resolutions, stopping the spread of weapons of mass destruction and global terrorism, even rebuilding Iraq after the invasion, are dismissed as "mandatory rhetoric".

Defence says the report is an "unofficial history" that represents the author's own views.