Winter is Coming
Not the good kind.
This planet has not had a nuclear holocaust. Yet.
The scary thing is people think we're safe because we haven't had one so far. However, though we have survived some seriously hairy bottle necks, there are even tougher times ahead.
Technology will progress to the point where a scientist in a bunker in Iran can design and manufacture a nuclear bomb that can fit into a suitcase. Or they can just download the blueprints and 3-D printer them into existence. Regardless of the method, the facts are this is inevitable - if we don't destroy ourselves another way first. Which is also scary.