This is not for the reasons emphasised by most critics – that a civilian trial is better than these men deserve, or that it will give them a platform for propaganda. The real problem is that the decision involves a needless risk, while failing to improve the legitimacy of the US government’s approach to terror trials.

Ideally, suspected terrorists should be tried under the same rules as people accused of other serious crimes. The horror of the offence is irrelevant. This should be obvious, but to judge by some of the attacks on Eric Holder, the attorney-general staking his career on the issue, it is not. The idea that a civilian trial is simply too good for the likes of KSM is indefensible, and that is all one can say.

The notion that a civilian trial will provide a platform for Islamist propaganda deserves to be taken more seriously. In the struggle to defeat jihadism, propaganda does matter. But in the end this objection is also wrong.

What could the accused say to win support from those not already dedicated to jihadism? The committed are committed, and that is that. Most of the rest would find an effort to glorify such astounding cruelty disgusting rather than inspiring. Propaganda-wise, there would be a countervailing effect. Refreshing the civilised world’s horror at the atrocities serves a useful purpose.

In explaining himself, Mr Holder makes a different argument about propaganda, while avoiding the term. And what he says is right, up to a point. Putting the accused through “the trial of the century”, as he called it, tells the world that the US holds itself to the highest standards of justice. Compare and contrast. See what these men did and look at our response. On one side, barbarians; on the other, civilised people.

But what happens – consider this possibility – if the defendants are acquitted? This interesting scenario is one that Mr Holder refuses to consider. Asked about it last week, while giving testimony on Capitol Hill, he said: “Failure is not an option. These are cases that have to be won.” So much for the presumption of innocence.

Clear as it might seem that Mr Mohammed and the others are guilty, acquittals contrary to a preponderance of evidence are not unknown. A civilian trial, with its stronger protections for the defendant, gives the accused’s lawyers more scope than they would have in a military commission to seek a technical acquittal, through the exclusion of tainted evidence and other stratagems. They have plenty of material to work with, starting with 183 episodes of waterboarding for the alleged chief conspirator.

Mr Holder believes the case against the five accused is rock-solid even with tainted evidence excluded. Maybe so. Mr Mohammed has not merely confessed to the crime, he has boasted about it. Perhaps he wants to be executed as much as Americans want to see him executed. But if he gets a good lawyer with an appetite for notoriety, and heeds his advice, he may have second thoughts. We shall see.

As well as discarding the presumption of innocence, Mr Holder is discarding the presumption that people acquitted of crimes will be set free. Officials have suggested that whatever happens, Mr Mohammed will not be let go: he will be detained indefinitely, regardless. So this exemplary trial either brings in the correct verdict, or else is annulled. This rather dulls the declaration of justice for all that the administration believes it is making to the world.

Most likely, the trial will turn out as Mr Holder intends, and the defendants will get their just desserts. Even in this best case, however, the legitimacy of the system for trying terrorist suspects will not have been improved, and arguably will have been further undermined – because having just declared them impaired, the Obama administration intends to keep using military commissions (and in some cases indefinite imprisonment without trial) for terror detainees.

Even as he announced the decision on the 9/11 plotters, Mr Holder said that the accused in the attack on the USS Cole would face a military trial. He had no satisfactory, or even intelligible, explanation why. The true reason, presumably, is that the case against the Cole accused is weaker than the case against the 9/11 accused. If in the special circumstances of a terror trial – which arise not from the severity of the crime, but from the difficulty of gathering and disclosing evidence – a military commission is a legitimate procedure, then it would have been legitimate for the 9/11 accused. If it is not, then it is illegitimate for the Cole accused.

If civilian courts cannot handle all such cases, the right course is to ensure that military commissions are fairly conducted, define the scope of the terror exception, then advocate that standard for all terror trials.

The administration is forum shopping. You can defend this as pragmatic. Use the best means available to put terrorists away, so long as you put them away – essentially, the Bush administration’s approach. You cannot defend it as legally consistent or high-principled. A civilian trial for the 9/11 plotters involves added risk and no judicial or political benefit. It is a mistake.

clive.crook@gmail.com