nuclear
Several nations possess nuclear weapons, with others seeking them.
Only the USA and Russia have arsenals large enough to trigger an extinction-level event.
Even smaller-scale nuclear conflict could cause global famine and environmental collapse.
The doctrine of Mutually Assured Destruction has prevented major conflict—so far.
Tactical nukes and unconventional delivery methods increase unpredictability.
Humanity has the power to destroy itself with the push of a button.
Historical ignorance and geopolitical instability make nuclear war a real threat.
Federation of American Scientists, NuclearFamine.org