Donald Rumsfeld has a habit of speaking his mind and damning the consequences, but his latest outburst - that there was no strong evidence linking al-Qaeda and Saddam Hussein - could have considerable consequences.
By Paul Reynolds
BBC News Online world affairs correspondent
At a critical moment in the presidential election campaign, with Democratic candidate John Kerry arguing that the war against Iraq was a "colossal error of judgement", the defence secretary has weakened one of the pillars on which the war was justified.
Donald Rumsfeld's frankness has got him into the headlines again
This pillar was the claim that Saddam Hussein represented an unacceptable threat because he had links with al-Qaeda and might give it or another group access to weapons of mass destruction.
The actual words Mr Rumsfeld used in his comments on Monday to the Council on Foreign Relations in New York were: "To my knowledge, I have not seen any strong, hard evidence that links the two."
He also said he had seen the intelligence "migrate in amazing ways", without explaining what that meant.
His statement was in marked contrast to what he said in September 2002 when he described the evidence of a link as "bullet-proof."
He did not however provide the evidence on that occasion.
After his latest comments, Mr Rumsfeld was forced to issue a statement saying that he had been "regrettably misunderstood."
He had, the statement said, always accepted the CIA evidence that there had been contacts between Iraq and al-Qaeda.
But the damage was done. While he had not said there was no evidence, he had suggested the evidence was not strong.
By questioning the significance of the intelligence, Mr Rumsfeld is undermining his own side and is handing an argument to the Kerry camp.
President Bush has often made a connection between Saddam Hussein and al-Qaeda and has made much of it. In his State of the Union address before the war, he said: "Evidence from intelligence sources, secret communications and statements by people now in custody reveal that Saddam Hussein aids and protects terrorists, including members of al-Qaeda."
And Vice-President Dick Cheney said in June that the evidence was "overwhelming" and accused the media of being "lazy" in not accepting this.
So what is the intelligence?
Part of it was laid out in an article in June in USA Today by Deputy National Security Adviser Stephen Hadley.
Quoting from a report compiled for the National Commission investigating the 11 September 2001 attacks, he listed the following:
Osama Bin Laden explored possible co-operation with Iraq during his time in Sudan.
A senior Iraqi intelligence officer reportedly made three visits to Sudan, finally meeting Bin Laden in 1994. Mr Cheney has additionally claimed that the officer was a brigadier general who trained al-Qaeda in bomb-making and document forgery.
Contacts between Iraq and al-Qaeda also occurred after Bin Laden had returned to Afghanistan.
Abu Musab al-Zarqawi (leader of the most extreme of the Islamist groups now operating in Iraq) and nearly two dozen al-Qaeda associates were in Baghdad before the fall of Saddam Hussein's regime. Zarqawi's network operated an explosives and poisons facility in north-east Iraq.
Mr Hadley made no mention of an earlier report that the 9/11 plot leader Mohammed Atta had met an Iraqi intelligence agent in Prague. This report still has its advocates but has never been confirmed.
The 9/11 Commission accepted that there had been contacts between Iraq and Osama Bin Laden but said there was "no evidence that these or the earlier contacts ever developed into a collaborative operational relationship".
Incidentally, Mr Rumsfeld also undermined the other pillar on which the case for war rested - that Saddam Hussein had weapons of mass destruction.
In his latest remarks he admitted that the intelligence had been "wrong."
He therefore switched from a position he had adopted only the previous day when he said in a Fox TV interview: "I believe they were there and I'm surprised we have not found them yet."
The Rumsfeld outburst was accompanied by an admission from the former US administrator of Iraq Paul Bremer.
While justifying the actual war, he said the US had "paid a big price" by not enough having enough troops there to maintain order.
He was thereby casting doubt on the judgement of the Bush administration - just as Mr Kerry has.
Mr Bush and Mr Cheney could have done without such remarks from Mr Rumsfeld and Mr Bremer at this time.