For nearly a century, the Anti-Defamation League has stared unflinchingly into the dark corners of America’s social psyche — the places where combustible tendencies such as hatred and paranoia pool and, sometimes, burst into flame.

As a Jewish organization, the ADL’s first preoccupation naturally is anti-Semitism, but in the last few decades it has extended its scrutiny to the whole range of bigoted malevolence — white supremacy, the militia movement, neo-nativism and conspiratorial fantasies in all of their improbable permutations. These days, the organization’s research is characterized by the sense of proportion and sobriety that long experience brings.

That makes its recent report on the extremist groups and propagandists that have emerged since President Obama’s election — “Rage…

With conspiracy talk-show host Alex Jones leading the way, one of the most disturbing trends in 2009 has been the resurrection and proliferation of some of the same anti-government conspiracy theories that so riled up domestic extremists in the 1990s. Such theories had been less popular in the 2000s, but Obama’s election gave anti-government conspiracy theorists a new focus for their efforts, a new light in which they could re-cast some of the old theories, and a willingness to come up with new conspiracies. More extreme than the “birther” conspiracy theory, these theories allege dark, violent designs on the part of the federal government.

Anti-Government Conspiracy Theories and Their Transmission

Though there are many conspiracy theories and numerous permutations, three are particularly entrenched among anti-government extremists:

• Imposition of Martial Law. The federal government is plotting to declare martial…