A CRITICAL distinction between this country and many of those with which the United States has clashed over the last century is that American citizens need not fear a knock on their door in the middle of the night that could lead to being whisked away and imprisoned without authorities having to justify their actions in court.

This is the constitutional line -- and the article of American faith -- that the Bush administration is dangerously running up against in indefinitely holding Abdullah al Muhajir, the U.S. citizen and alleged al-Qaida terrorist, incommunicado and without charges, a trial or legal representation in a military prison.

It is a protection that the president, Congress and federal courts must find ways to uphold, rather than breach -- in this particular case and in similar ones likely to come along.

True, the U.S. Constitution, in the words of one prominent legal analyst, is not a "suicide pact," and does not require Americans to sacrifice their nation's security in the name of protecting individual rights. But, of course, the enduring strength of the Constitution is that it serves as a brake on those who would quash citizens' basic rights in the name of such fears.

And these days, there's a lot to be scared about. Mr. Muhajir's alleged intent -- to set off a "dirty bomb" that would spread radiation somewhere in the United States -- is a dangerous scenario that resonates with this nation's fears since the terrorist attacks of Sept. 11. If true, he should be aggressively prosecuted in federal courts and punished. In the meantime, given that there well could be more to the alleged plot than just that, it's not surprising that authorities would want to keep him locked away and isolated in a kind of preventive detention.

But a federal court already has ruled that Mr. Muhajir, arrested more than a month ago, couldn't be held as a material witness in a federal investigation of al-Qaida. So the administration -- perhaps with insufficient evidence to bring charges against him -- designated him an "enemy combatant" and transferred him to Navy custody in South Carolina, even though U.S. citizens aren't to be tried before largely secret military tribunals. So this case -- and perhaps that of another American captured in Afghanistan and now being held by the Navy in Virginia -- amounts to detention by presidential fiat, one that theoretically could last forever.

This is a tempting legal device given that America now is engaged in a new kind of war, one being fought on our doorsteps as well as abroad and one in which the enemy might be anywhere. Rightfully, Americans are scared. A national poll this week shows that most are willing to sacrifice some of their privacy rights for a greater sense of security. Moreover, the administration already is under scrutiny about whether federal authorities were vigilant enough about homeland security before last fall.

But the fundamental issue in this case goes far beyond willingly trading some privacy protections for greater security. Mr. Muhajir's military detention is a serious problem because it runs directly counter to American values, to why Americans have so often fought against tyranny in the first place. It is not the way out of the vexing bind of how to protect this nation while maintaining the rule of law. If the administration and Congress cannot figure out how to do that, then Americans have more to fear than just terrorism.