first strike

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A preemptive attack against an enemy, especially one using nuclear weapons against an enemy armed with nuclear weapons.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the initial use of nuclear weapons to attack a country that also has nuclear weapons; considered feasible only when the attacker can destroy the other country's ability to retaliate