Let's start with what Hit and Run is not.

It is not a sequel to Nicky Hager's last bombshell, Dirty Politics.

Mercifully, the word "blog" does not, I don't think, appear even once in the 120-odd page length of the book, subtitled "The New Zealand SAS in Afghanistan and the meaning of honour", which documents a disastrous 2010 raid, led by New Zealand, with US and Afghan forces, in Baghlan, the province neighbouring New Zealand's Bamiyan base.

It is not an election campaign coup de theatre; it is not the "real reason John Key resigned" thunderbolt that those who see horns sprouting from the former prime minister's hairline so craved.

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It was not timed - as anyone who has been involved in publishing a book can tell you - to disrupt the eve of Key's valedictory speech.

Hager and his co-author, Jon Stephenson, have stressed both these points.

The then prime minister did sign off the raid, which apparently killed six civilians and injured at least 15 more, but there is no claim that he masterminded any coverup.

"I suspect we know far more about what happened than John Key was told," said Hager.

Hit and Run is not some cooked-up, conspiratorial yarn. It is not "bogus crap", as National MP Paul Foster Bell tautologously described it in an embarrassing Twitter tirade late on Wednesday night.

Paula Bennett, the deputy prime minister, seemed about to call Hager a "leftwing conspiracy theorist" on the radio the other morning, but abandoned the slur one syllable into the second word, perhaps thinking, given the nature of the allegations in the book, that line may not wash this time. Or maybe she was going to say leftwing conscience and critic.

The prime minister Bill English has not, as far as I know, used the term his predecessor favoured, either. He did describe the book as "politically motivated", a remark that can reasonably be characterised as, well, politically motivated. But to his credit he has left the door open to an independent investigation.

The book is not suggesting that the New Zealand elite forces involved in the awful events of August 2010 set out to hurt civilians.

It does make the case, however, that the attacks on Naik and Khak Khuday Dad, neighbouring villages in the Tirgiran Valley, in retaliation for the appalling killing of a New Zealand soldier, were overhastily planned, predicated on bad intelligence, poorly executed and ultimately bungled. It details 21 civilians killed or injured, most of them seemingly struck by gunfire from US attack helicopters operating in support of a New Zealand led and commanded raid.

It is not disputing that the New Zealand Defence Force is replete with morally upstanding and courageous men and women.

Indeed, it seems precisely in the service of those values that sources have spoken to Stephenson and Hager about what they perceive as a cover-up, not just of the civilian deaths, but of failure to assist the injured and the transfer of a prisoner into the hands of an apparent torture squad.

It is not ignoring the fact that six civilian deaths is a drop in an ocean of innocent bloodshed in Afghanistan and around the world.

That hundreds of thousands of innocent people have been killed already this century in Afghanistan and Iraq, in Syria, Yemen and elsewhere, does not make this story less important, but more so - precisely because it is unconscionable in our name, precisely because we are told their names, about their lives as human beings.

In the case of Fatima, killed by shrapnel from the NZ-led attack, we see her photograph, her bright blue dress, her toes poking out of sandals too big for her feet, plump-cheeked, grinning a 3-year-old's grin.

Fatima's mother was hospitalised, her two brothers wounded. One of them, Abdullah, then 7, said when interviewed last year, "When we have cold weather the pain in my head gets worse. We become angry and upset when we remember that time, but what can we do?"

The human misery is compounded by the knowledge that a civilian death is a recruiting gift to insurgent groups like the Taliban. US generals have stressed this point. So have their enemy.

In 2010, around the time of the Baghlan attack, former Taliban spokesman Mullah Abdul Salam Zaeef told Jeremy Scahill, investigative journalist and author of Dirty Wars, that the raids, claiming so many innocent lives, "are encouraging people to become extremist ... When you are killing one person, four or five others rise against you."

Today, the Taliban remain active across swathes of Afghanistan, including Baghlan. Last month, they seized control of a second district in the province.

In response to the publication of the book, the NZ Defence Force issued a statement standing by an earlier release in 2011, which insisted that, based on an investigation conducted by the Afghan ministry of defence and Isaf, the Nato-led security mission in Afghanistan, "the allegations of civilian casualties were unfounded" and that "nine insurgents were killed in the operation".

In the face of compelling evidence that the insurgents had fled to the mountains, expecting a reprisal, in the face of documentary evidence of civilian deaths, that is now untenable.

It is untenable, further, given the remarks of Wayne Mapp, the defence minister at the time. He yesterday told RNZ, "the mission did not achieve its objectives ... the people we were aiming to get, we did not get". He confirmed he had previously described the raid as "our biggest and most disastrous operation. A fiasco."

Mapp maintained that the soldiers had acted reasonably, and had responded to a threat.

But, he said, he had not known a 3-year-old was among the civilians killed, indeed that any civilians had been killed, and acknowledged there now was compelling evidence to that end.

He is not some casual observer. He was the minister of defence. The minister of defence has the statutory "power of control of the New Zealand Defence Force".

It is inexcusable, to put it mildly, if information of this gravity was withheld from him.

Hit and Run is an important book. Whether you admire or viscerally loathe its authors is immaterial to the evidence it documents.

Not all of the allegations are new, but the depth of research and detail are compelling.

Any journalism that heavily depends on unnamed sources should, of course, be subject to scrutiny, even if, as here, they are numerous and corroborated.

Critically, many of the sources would be willing to speak to an appropriate, independent investigation, says Stephenson.

For their sake, for the sake of the NZ Defence Force, whether to censure or vindicate, for the sake of the government, for the sake of respecting international law, for the sake of the dead, and in the public interest, that investigation needs to happen.

Not to do so for fear of creating difficulty for our military bosses or politicians or, even, the Americans, would be wrong.

"We're not going to be rushed into an inquiry," was an early response from the prime minister, and that is fair enough, but the case is now urgent and overwhelming.