Today, by ordering a military trial at Guantanamo for 9/11 plotter Khalid Sheikh Mohammed and his co-defendants, Attorney General Eric Holder finally put the Obama administration’s stamp on the proposition that some criminals are “too dangerous to have fair trials.”

In reversing one of its last principled positions—that American courts are sufficiently nimble, fair, and transparent to try Mohammed and his confederates—the administration surrendered to the bullying, fear-mongering, and demagoguery of those seeking to create two separate kinds of American law. This isn’t just about the administration allowing itself to be bullied out of its commitment to the rule of law. It’s about the president and his Justice Department conceding that the system of justice in the United States will have multiple tiers—first-class law for some and junk law for others.

Every argument advanced to scuttle the Manhattan trial for KSM was false or feeble: Open trials are too dangerous; major trials are too expensive; too many secrets will be spilled; public trials will radicalize the enemy; the public doesn’t want it.

Of course, exactly the same unpersuasive claims could have been made about every major criminal trial in Western history, from the first World Trade Center prosecution to the Rosenberg trial to the Scopes Monkey trial to Nuremburg. Each of those trials could have been moved to some dark cave for everyone’s comfort and well-being. Each of those defendants could have been tried using some handy choose-your-own-ending legal system to ensure a conviction. But the principle that you don’t tailor justice to the accused won out, and, time after time, the world benefited.

Now the Obama administration—having loudly and proudly made every possible argument against a two-tier justice system—is capitulating to it.

But make no mistake about it: It won’t stop here. Putting the administration’s imprimatur on the idea that some defendants are more worthy of real justice than others legitimates the whole creeping, toxic American system of providing one class of legal protections for some but not others: special laws for children of immigrants, special laws for people who might look like immigrants, different jails for those who seem too dangerous, special laws for people worthy of wiretapping, and special laws for corporations. After today it will be easier than ever to use words and slogans to invent classes of people who are too scary to try in regular proceedings.

Say what you want about how Congress forced Obama’s hand today by making it all but impossible to try the 9/11 conspirators in regular Article III courts. * The only lesson learned is that Obama’s hand can be forced. That there is no principle he can’t be bullied into abandoning. In the future, when seeking to pass laws that treat different people differently for purely political reasons, Congress need only fear-monger and fabricate to get the president to cave. Nobody claims that this was a legal decision. It was a political triumph or loss, depending on your viewpoint. The rule of law is an afterthought, either way.

It may not matter to you today that the U.S. government has invented a new class of criminals fit for a new class of trials. It may bother you a lot more when special rules are created for unions, or corporations, or the poor, or the children of illegal immigrants, or eco-terrorists. Today’s capitulation will just embolden Congress to do all that and more.

A year ago, Holder told the New Yorker’s Jane Mayer that the trial of Khalid Sheikh Mohammed would serve as “the defining event of my time as attorney general.” Sadly, he’s probably right. He will be remembered for having sacrificed what he knew to be right for some payoff to be named later. We will, all of us, in the long run bear the costs of that choice.

Correction, April 5, 2011: This sentence originally referred to Article II courts. (Return to the corrected sentence.)