We don’t know whether this latest warning is a false alarm or not. Let’s hope it is. As anyone who has lived through a long terrorist campaign will know, false alarms come with the territory. Just ask Londoners familiar with the false alarms that regularly emptied tube stations for years. A nuisance? Yes, but an inevitable part of responding responsibly to a very real threat. What a pity so many of the White House’s critics do not appear to understand that, in times like this, erring on the side of caution is the right thing to do. Not doing so could be lethal.