The explosive Commander-on-Chief forum on national defence for America's presidential candidates triggered sharp criticism of NBC's moderator for letting Donald Trump dodge questions — and, more importantly, blatantly contradict at least one verifiable fact.

The forum raised concerns about what exactly the media should do to best serve viewers during the upcoming debates if a candidate presents a lie as the absolute truth.

It appears to be a particular problem when it comes to Donald Trump, who is a candidate unlike any the US has ever seen before.

The issue arose when Trump insisted to NBC moderator Matt Lauer that he has been opposed to the US war in Iraq right from the start. In fact, in an audio clip on a 2002 radio programme Trump is asked point-blank if he supports the war in Iraq.

"Yeah, I guess so," he says. "I wish the first time it was done correctly."

Lauer failed to challenge Trump's rewriting of history and the hashtag #laueringthebar was born.

New York Times columnist Nick Kristof called Lauer's lapse an "embarassment to journalism", while former Wall Street Journal editor Bill Grueskin called it "a master class in bad interviewing."

Behind the scenes, at least one NBC executive deemed Lauer's interview a "disaster," reported CNNMoney.

Buzzfeed tracked down another interview in 2004 with Fox News in which Trump called the US invasion of Iraq a "tremendous success from a military standpoint".

Not only does Trump insist he was against the war from the start, he claims he was contacted by White House officials to keep his mouth shut about his opposition.

The Washington Post Fact Checker feature, which has consistently tracked truth and falsehoods in the campaign, has presented Trump's own words about the war contradicting his later assertions — and hasn't located a single person in the White House who complained about Trump's early opposition to the war, which didn't exist in any case.

The day after the forum, several media organisations emphasised the same issue: Trump lied and Lauer let him get away with it.

Trump, as he often does, doubled down in his response to the storm of controversy, and slammed the media for reporting his manipulation of the facts. He said at a campaign stop at a school in Cleveland that his comments later in which he switched his position and opposed the war "superseded" — and apparently erased — his earlier comments, the Cleveland Plain Dealer reported.

But the problem remains: What can and will media moderators do during the crucial upcoming live candidate debates when a lie arises and fact checking on the spot may be difficult and even impossible? (The first one will be moderated by NBC's Lester Holt 26 September.)

More troubling, notes Fortune magazine, is a journalism culture focused so scrupulously on "balance" that reporters can tend to simply report or allow lies rather than to challenge them.

"That kind of principle makes sense when you're dealing with a relatively normal political environment, in which most of the candidates stick to an unspoken agreement not to lie or simply make things up, or make outrageous statements that have no bearing on reality," notes Fortune.

"Trump, however, demonstrably and repeatedly breaches this principle, and that is part of what gives him so much power and influence — especially when the traditional media reports his statements verbatim without bothering to express any opinions about whether they are correct or not, or whether they even make sense."

It's a problem the "media as a whole deserves the blame for—not just Matt Lauer," notes Fortune.