Bitcoin uses electricity to make transactions so you answered your question there no power no bitcoin.
Now days power is literally everywhere around us. Even if someone, somehow shut down entire system nothing will be lost, everything will be kept on hard drives, and after plugin in again everything will be restored and value can increase. No power just means you can`t transfer bitcoins, but no power doesn`t mean there is no bitcoins, they don`t depend from electricity.
The third world war? If the outbreak really, I think this time you should not be worried about bitcoin prices down, you should worry about their own lives. When people began to consider the life safety, bitcoin will probably not worth a hair.
Exactly my point, in the war times people think about their lives, but there is always war profiteers, they will go from house to house and they will take everything worth they find, I know that I have lived in war more then 7 years.