The U.S. military expects to have 20,000 uniformed troops inside the United States by 2011 trained to help state and local officials respond to a nuclear terrorist attack or other domestic catastrophe, according to Pentagon officials.

The long-planned shift in the Defense Department's role in homeland security was recently backed with funding and troop commitments after years of prodding by Congress and outside experts, defense analysts said.

There are critics of the change, in the military and among civil liberties groups and libertarians who express concern that the new homeland emphasis threatens to strain the military and possibly undermine the Posse Comitatus Act, a 130-year-old federal law restricting the military's role in domestic law enforcement.

The American Civil Liberties Union and the libertarian Cato Institute are troubled by what they consider an expansion of executive authority.

Domestic emergency deployment may be "just the first example of a series of expansions in presidential and military authority," or even an increase in domestic surveillance, said Anna Christensen of the ACLU's National Security Project. And Cato Vice President Gene Healy warned of "a creeping militarization" of homeland security.

"There's a notion that whenever there's an important problem, that the thing to do is to call in the boys in green," Healy said, "and that's at odds with our long-standing tradition of being wary of the use of standing armies to keep the peace."