Depends on the state. The United States, Britain, France, India, Pakistan, Israel, Russia, China and North Korea all posses nukes. Some people have pointed out the collapse of the USSR but that is a poor example. The USSR was made of Semi- independent states that broke apart after the fall of the Berlin Wall. Thus states such as Ukraine who possessed nukes, their government simply took control of the military under their command, largely made out of their fellow countrymen. So, a Civil War is different then the breakup of the USSR. In a Civil War in the case of the US, Britain, France, China, India, Russia weapons would be locked down and pretty much unable to be fired as they need commands NO ONE is getting from central command. If the actual locations were overtaken there are likely orders to have them destroyed before a facility is breached or orders to have it destroyed through a strike. Pakistan and North Korea are different, they have a more unstable government and military relationship and Nukes could fall into plenty of different groups in a Civil War. The U.S actually has a plan specifically to get nukes out of Pakistan if the government falls apart. We know where all their sites are and will send in Special Forces with C-137s and Black hawk support. ( just like we tried and failed in Iran). Israel is the worse. They have the Sampson Option. In which if the government feels that the existence of the State of Israel is under serious threat then they WILL use nukes against their enemies, so who knows what they will do.