US ground forces against a country 1/3 the size of China with 90 million people is guaranteed to be a US military defeat, so Iran just realized they were actually stronger than they though. NK looked at all this, and had zero issues in deploying 10,000 combat troops in active combat in Europe, in case you have not paid attention to Ukraine...
If you use even one single nuclear weapon, either as North Korea or Iran, you can be assured of being turn to ashes in the next minutes. So they dont make sense as a weapon, they are more of a status symbol.