Throughout the long reign of J. Edgar Hoover, the F.B.I. voraciously compiled information on suspected national-security threats -- and often put it to damaging use. Beginning with a campaign against the Communist Party, U.S.A., in 1956, the bureau's notorious Cointelpro programs aimed to ''expose, disrupt, misdirect, discredit or otherwise neutralize'' their targets. The nearly continuous monitoring of John Kerry's antiwar activities in the early 70's only hints at the sweep of surveillance efforts at the time.

While the F.B.I.'s campaign against antiwar ''subversives'' was largely ineffective, a considerably less ardent campaign against the Ku Klux Klan and its allies proved devastating. In the late 60's and early 70's, membership in white hate groups dropped as much as 70 percent; paranoia over infiltration reached such heights that the national Klan leader, Robert Shelton, threatened to use polygraph tests and truth serum to gauge members' loyalty. While the F.B.I. sought to dismantle the new left, it merely hoped to control the white right. And yet its activities did far more damage to the racists than the radicals. Is there a lesson to be learned from this arguably unintended victory?

When three civil rights workers were murdered in Mississippi during the ''freedom summer'' of 1964, Attorney General Robert Kennedy and others in the Johnson administration pushed hard for the F.B.I. to take on a new foe: the militant white defenders of segregation. Hoover reluctantly responded by opening a bureau field office in Jackson, Miss., and transferring 153 agents to the state. Behind the scenes, he also took aim at 26 so-called white hate groups in the South.

The ''Cointelpro-White Hate Groups'' program was not any more aggressive or imaginative than the bureau's campaigns against civil rights or antiwar protesters; in all cases, anonymous letters, phone calls to parents or employers, the planting of derogatory news stories and the covert use of informants were the F.B.I.'s tactics of choice.