When it comes to making sure the American public doesn't get complacent about the threat of a terrorist attack, no one can accuse Michael Chertoff of sitting on the sidelines. The nation's Homeland Security secretary has regularly sounded a warning cry, as he did again last week.

The irony is that these warning cries may, of themselves, be fueling complacency.

A year ago, Chertoff raised jitters when he told the Chicago Tribune editorial board he had a "gut feeling" there was a heightened risk for a terror attack during the summer. Such an attack, of course, did not happen.

Now Chertoff has told Congress that terrorists with European Union passports, and with clean criminal records, may be attempting to enter the country. He offered few other details and said nothing about specific threats.

Chertoff isn't the only one talking this way. In March, Air Force Gen. Gene Renuart, chief of the U.S. Northern Command, said al-Qaida may be working extra hard to plan an attack coinciding with this year's presidential elections. He also said he believes terror cells are operating within the United States.

Always, there is talk of terrorist "chatter" overheard by U.S. intelligence officials. But never are specifics shared with the public. It's easy to see why Americans might grow skeptical or tired of the repeated warnings seven years after the last attack.

Such is the nature of this frustrating war against an opponent guided by ideology and hatred, rather than national identity. Anyone who doubts the relentless nature of this enemy needs only to remember that the attacks of 9/11 took years of meticulous planning. U.S. intelligence may indeed have thwarted numerous attempts to carry out attacks since then, but the terrorist mind-set is unbowed by failure. In a spirit worthy of the best door-to-door salesman, terrorists know the harder they try, the better the chances of succeeding somewhere.

Chertoff has little to gain personally from continually sounding alarms. Despite rumors, he is an unlikely choice to be John McCain's running mate. His term as Homeland Security chief ends when President Bush leaves office in January.

The American people, however, have much to gain from remembering the way they felt in the days following 9/11, and to be continually alert to anything that seems out of the ordinary.