Within three years, the Defense Department plans to have 20,000 troops, in three rapid-reaction forces, trained and ready to respond to terrorist attacks or other catastrophes on U.S. soil.

In these parlous times, and especially with images of the Mumbai massacre still so vivid, it seems eminently reasonable to have military forces trained to react internally. Indeed, members of the Bush administration and others have warned for years that one of the greatest threats facing the United States is a terrorist attack involving a dirty bomb or some other easily portable weapon of mass destruction of the type said to be proliferating around the world.

Eminently reasonable, yet somewhat unsettling.

Civil liberties groups and others have expressed concern that such a use of the military in homeland security duties could violate the Posse Comitatus Act, a 130-year-old law that restricts the use of federal troops in domestic law enforcement.

We share those concerns. Since the time of the Founding Fathers, Americans have been uneasy about the role of the military in domestic matters. Nonetheless, there is a long history of using troops to deal with domestic matters, from President George Washington's effort to quell the Whisky Rebellion of 1794 to the Los Angeles riots in 1992 to the aftermath of Hurricane Katrina.

Pentagon homeland defense plans call for the ability to respond to "multiple, simultaneous mass casualty incidents." The teams will be specifically trained to deal with domestic chemical, biological, radiological, nuclear and high-yield explosive attacks. More than $556 million will be spent over several years to set up the teams, which are envisioned responding to disasters involving evacuations of 1 million people or more, with thousands of casualties and contamination of up to 3,000 square miles.

So, while we share some of the unease expressed by civil libertarians, we are confident that careful oversight can ensure that the use of the troops conforms with the law. With that issue addressed, it would be irresponsible not to have well-trained military units ready to assist local and state authorities in dealing with the horrifying prospects that confront us.

REPRINTED FROM THE SAN DIEGO UNION-TRIBUNE.