why should other nations not?
Al Qaeda is not a nation. Would you agree that the Sicilian Mafia has ultimate authority over the Swedish people, and can do as it pleases on Swedish territory?

Surely, if the USA can send civilians on a killing spree to foreign countries, why should other nations not?
As in, merely a conventional war? Any nation can of course try to circumvent international law, international attitudes, and reason itself and directly attack the American mainland - they wouldn't last long.