I do not think one nuclear strike would automatically start global hellfire, but it could be the beginning of the end for that nation’s place in the world order. Much would depend on how the weapon was delivered and who was targeted. If it came by long range ballistic missile, the other side would have only minutes to decide whether it was a single strike or the start of something much larger. If they chose to launch what they had in response, God knows where it would end. The most important question is, where do you want to be if that ever happens?