For perhaps 72 hours after 8:45 a.m. Sept. 11, 2001, Rudy Giuliani played the role of acting president of the United States.

Of course, the man who was then the mayor of New York had no formal national authority. But from the moment the terrorists attacked, Giuliani was the face of national resolve, the angry but calm voice reassuring Americans -- in Rocky Mountain hamlets no less than on the streets of the Rockaways -- that their country was brave, that it would survive, that it would eventually triumph.

Rudy, as all of us came to know him, lived up to a responsibility that fell to him by default. While the mayor rallied the firefighters and the cops and the rescue workers, President Bush found himself far from the action. He was visiting a school in Florida when the planes hit the World Trade Center and the Pentagon. As The Post put it at the time, he then "boarded Air Force One and, escorted by fighter jets, hopscotched to military installations in Louisiana and Nebraska before returning to Washington." Bush's speech to the nation that night was flat. David Frum, a former Bush speechwriter, noted in his largely laudatory book "The Right Man" that the president "had given not one indication all day long of readiness for his terrible new responsibilities."

Bush finally seized the moment three days after the attacks when he visited the Trade Center site and shouted to a rescue worker: "The people who knocked these buildings down will hear all of us soon." The president finally seemed ready and soared in the polls. Until then, Rudy Giuliani was the de facto spokesman for a grateful nation.

All of which makes you want to ask Giuliani why he felt it necessary to rebuke the commission investigating Sept. 11 for pointing out important truths about what went wrong that day. In the matter-of-fact way of its valuable staff reports, the commission pointed to the turf battles and communication problems among New York City's uniformed services that may have cost lives.

Rudy, no one is asking you to be perfect. No one, and I mean no one, is taking anything away from the bravery of those who selflessly gave their all that day. But the Sept. 11 commission has the responsibility for making us more ready if a dreadful event of this sort happens again. They can't overlook what went wrong.

Alas, most things are personal for Rudy. "Our enemy is not each other," he told the commission on Wednesday, "but terrorists who attacked us, murdered our loved ones and continue to offer a threat to our security." Of course that's right. But no one says you're the enemy, Rudy. Yet none of us, certainly not you, would want systems kept in place that threaten the very men and women whose bravery protects us.

Most of the commission members seemed thoroughly intimidated by Giuliani and expressed their devotion. It fell to Bob Kerrey, the Vietnam veteran who does not intimidate easily, to state an important truth: "I don't believe it's an either/or choice of being angry at those who perpetrated this crime and feeling anger towards those with responsibility."

Kerrey went on to praise Giuliani too, but his point goes to the heart of the commission's challenge and mandate: to overcome the mythologizing of Sept. 11 and face what happened.

If, indeed, some firefighters died inside the World Trade Center because they did not hear an evacuation order, Giuliani does not have to deny the fact, as he did in his testimony. Let's assume Giuliani really does believe they stood their ground in order to rescue civilians. The commission has no choice but to deal with all the evidence that points instead to those communication and coordination problems.

And evidence is what should matter to this commission. It simply can't allow those with an interest in having the Sept. 11 story told a certain way to get in the way of telling us the real story. That means especially telling the story of what went wrong. That's the only way we'll learn how to do things right.

Rudy, listen to Bob Kerrey. You have nothing to fear from an honest account. As you acknowledged yourself, "some terrible mistakes were made," which were, as you also said, inevitable in these excruciating circumstances. Americans came to admire you because they saw you as a tough truth-teller. Don't let worries about your image now tarnish the image you've already earned.