He agreed with New Zealand First MP Ron Mark's claim in December that troops were already training in Waiouru for desert warfare.
Dr Buchanan said that after Mr Mark's comments, troops were gagged from talking about Iraq on penalty of court martial.
He said the visit of Iraqi foreign minister Dr Ibrahim al-Ja'afari earlier this month added "diplomatic gloss" to what was already planned.
The Government needed to ditch the "fig leaf" and admit the deployment was a done deal, Dr Buchanan said.
"Just be honest. We've got to go. We're not there to pussyfoot around."
He said the New Zealand public should be prepared for casualties. The question was, he said, how many casualties the public could tolerate before demanding withdrawal.
"This is going to be a very nasty, door-to-door, bloody conflict," he said.
Labour defence spokesman Phil Goff said it seemed Mr Key had privately decided months ago to deploy troops to fight Isis.
He said New Zealand's Western allies, rather than the Iraqi government, were driving the push to send Kiwi troops to the Middle East.
"My problem, and the Labour Party's problem, is the avenue Key has chosen is likely to be the least effective way of dealing with the problem."
He said that was because the Iraqi army was corrupt, had a "pathetic" leadership and was itself a cause of sectarian tensions and subsequent grievances Isis used to win support.
Mr Goff said Isis needed to be contained and isolated, starved of funds, weapons and personnel, and its victims given help.
Defence Force personnel he had heard from were ambivalent about deploying to Iraq, with more experienced veterans the least jingoistic.
Associate Professor Stephen Hoadley, international relations scholar at the University of Auckland, said the deployment of Kiwi troops seemed inevitable.
"The question of precisely where they're going to be, and what their security will be, is top of the agenda."
He said it was unlikely more than 100 troops would go to Iraq, and probably far fewer would be there at any given time.
Dr Hoadley said a parliamentary select committee would later debate the deployment but the Government would use its prerogative to deploy troops anyway.
He said National would sustain domestic political hits if an Iraqi quagmire emerged, especially if National bypassed a vote in Parliament.
"It will bite them in the future if the deployment goes sour, if somebody gets killed, or if the Iraqi government refuses to reform itself and appears to be an unworthy recipient of New Zealand's assistance."
National was far from persuading all its partners of the deployment's merits.
United Future leader Peter Dunne said he was firmly opposed to deploying troops, despite Mr Key entreating him to back the action.
"We each have a different view. It's fair to say he accepts, but not agrees, with my position. And I suppose I do likewise with his."
Confidence-and-supply provisions between the two parties did not cover the deployment, Mr Dunne said.
"It's a judgement call for any government to make. In this instance I think it's the wrong call."
