Walzer’s concept of a supreme emergency is unclear and vague. It is very hard to distinguish genuine supreme emergencies from other cases in which threats of terrible destruction and damage exist but do not justify terrorism. Is a threat of mass enslavement or extermination a necessary feature of any supreme emergency? Or can other threats to civilized values qualify? Or could there be enslavements and exterminations that do not threaten civilized values? These questions all weaken the theory of supreme emergencies.
The example in the argument about the British bombings of German cities does not constitute a supreme emergency that could justify the terror. Britain, as a whole, never faced a threat of enslavement or extermination. While the Nazis explicitly sought to exterminate Jews and enslave Slavs, they took no steps to enslave or exterminate the French or the Scandinavians. Their racist ideology provided no reason to think that genocide or enslavement awaited the British. Therefore, Britain only faced a supreme emergency of political disaster and not one of a moral disaster.
Even if the British example is assumed to be valid, supreme emergencies are extremely rare. It is not a familiar principle that people understand and know how to apply. There is no evidence that people involved in the World War II city bombings thought in this way. Had they reasoned as Walzer does, they would have stopped city bombings when the supreme emergency was over.