The arrest of two leaders of an Albany mosque last night in connection with an attempt to aid in the purchase of a shoulder-mounted surface-to-air missile is a reminder that vigilance is needed by all New Yorkers. According to prosecutors, the two men — the imam of the Masjid As-Salam mosque, Yassin Aref, and the mosque’s founder, Mohammed Hossain — helped someone they thought was a terrorist buy a missile. But the “terrorist” was really an informant who was cooperating undercover with federal law enforcement officers.

We’ve long been skeptical of sting operations, which can easily veer over the line into entrapment. Remember the Abscam bribes of congressmen in 1980 or Washington mayor Marion Barry with cocaine in a hotel room in 1990?

In a free society, government’s job is to prevent crime and punish it, not to lure citizens into committing crimes that they otherwise would not have intended to perpetrate.

Such legal issues in this case will no doubt be a matter for courts to consider. The Albany Muslims are entitled to the presumption of innocence like anyone else. But news that there are persons in America who would traffic with those in the market for a shoulder-mounted missile that was said to have been for use against the Pakistani government mission in New York undermines the idea that the recent vigilance against terror in America is all just hype.

Imagine what you would do if somebody showed you a shoulder-fired missile and told you he intended to use it against a site at 44th Street and First Avenue in New York City. The deputy attorney general, Jas. Comey, suggested yesterday that prosecutors will pose that question to the jury. We’re not particular fans of the military government in Pakistan, but it’s more than a little alarming that persons would engage in financial transactions with someone who declares he is planning an attack with a missile in the middle of Manhattan.

Another point is that — contrary to the objections of some civil libertarians and Muslim Americans — some people who are ready to violate the law and who bear watching by federal authorities are to be found in mosques. This does not mean that guilt or suspicion should or will attach to all mosques or to all Muslims. But it does mean that when there is a probable cause, it is perfectly proper for the FBI to maintain its vigilance in a war that is going to last for years and test America’s mettle as never before.