However, experience questions whether such generalized warnings should be issued at all. If intelligence has uncovered specifics of target and time of attack, the sensible response, of course, is not to bloviate grandly, but to work to secure the likely target or targets or to use policing measure to disrupt the plot.
But if, as it appears thus far in the present case, the warnings are vague and unspecific, issuing proclamations of danger out of an “abundance of caution” (as it has been put by the State Department) scarcely helps the situation. It is not clear what individuals are supposed to do except perhaps to look over their shoulders more often, and the proclamations often exact substantial costs in unnecessary anxiety, disruption, inconvenience, and lost economic opportunity. Nothing wrong, I suppose, in telling Americans that Yemen is not the safest travel destination. But few are likely to need the warning.
At any rate, the record strongly suggests that warnings based on vague and unspecific intelligence have failed to stop or to protect against terrorist attacks. Conceivably, some attacks have been deterred by the proclamations, but, if so, one would expect truly dedicated terrorists, inconvenienced in their immediate planning, to move into action once the alarms have faded. But that, it seems, hasn’t happened.