The government can't possibly defend Americans against every possible terrorist target every moment. Yet that is the challenge faced by Homeland Security Secretary Michael Chertoff, and it is why his recent statements about priorities struck some as cruel.

Chertoff said the government should concentrate mainly on preventing further attacks via the air. Cities, on the other hand, are mostly on their own as far as preventing the type of mass-transit disaster that struck London. That philosophy was reflected in a subsequent spending bill Congress passed, granting $31.8 billion for homeland security but reducing the amount for rail and transit safety by $50 million.

Whether Chertoff ends up a hero or a fool will depend, of course, on what plots terrorists may be hatching.

What money he does direct toward mass transit, however, ought to be targeted at places that are most vulnerable.

It would make little sense, for instance, to spend a lot of time or money increasing security on TRAX here in Utah. Most of the attention should focus on more likely targets, such as transit systems in New York City and Washington. Even other large cities, such as Atlanta or Chicago, probably should be treated as less-likely targets.

Even then, there are limits to how secure a public transit system can become. London already used a lot more public funds to secure its system than the United States is proposing, and that didn't prevent the tragedy that occurred there.

Americans quite naturally have turned their attention toward mass transit in the wake of those bombings, just as they were more focused on air travel after 9/11. A recent poll by the Associated Press and Ipsos found that 57 percent of Americans believe an attack on a bus, subway or train is inevitable, and nearly two-thirds believe it can't be prevented.

The problem, of course, is that no one knows for sure where the next attempted attack will be targeted. Nor do Americans really know how many such attacks may already have been thwarted. The best defense is to continue focusing on gaining intelligence into possible terrorist cells and plans that may be in the works.

In that regard, Americans have little choice but to trust people like Chertoff, who, one hopes, has enough inside information to intelligently allocate resources.