Fear of the 'enemy within'
The fear of the "enemy within" is as old as history, but it is what adds a frisson of fear to the arrest of Faisal Shahzad in connection with the Times Square bomb plot.
He was born in Pakistan and spent five months there recently, but last year became an American citizen.
Of course, in any country, in any city, there would be a sense of shock if a potential car bomb, however bungled and amateurish, was found in a major city. But in America, in New York, the alarm is greater, because of 9/11 and because of the determination after that murderous assault that the United States would never again be attacked on its own soil.
Those of us who have lived with IRA bombs in the background to our daily lives know that that can be an aspiration, but not a promise any politician can keep. Particularly if the suspected enemy is a fellow citizen.
President Barack Obama has said that Americans should not live in fear, or be cowed. So far, there have been none of the political mutterings that followed Fort Hood and the underpants incident (or at least I haven't spotted them) and so maybe it is only a matter of time before the point-scoring starts up again. Or maybe the US is learning that with all the vigilance, the best intelligence, the tightest security, you can never eliminate all risk, and that the unwitting allies of terrorists are those who want America to be terrified.