First strike (nuclear strategy)
In nuclear strategy, a first strike or preemptive strike is a preemptive surprise attack employing overwhelming force. First strike capability is a country's ability to defeat another nuclear power by destroying its arsenal to the point where the attacking country can survive the weakened retaliation while the opposing side is left unable to continue war. The preferred methodology is to attack the opponent's strategic nuclear weapon facilities (missile silos, submarine bases, bomber airfields), command and control sites, and storage depots first. The strategy is called counterforce.
Because of the low accuracy (large circular error probable) of early generation intercontinental ballistic missiles (and especially submarine-launched ballistic missiles), counterforce strikes were initially only possible against very large, undefended targets like bomber airfields and naval bases. Later generation missiles with much improved accuracy made counterforce attacks against the opponent's hardened military facilities (like missile silos and command and control centers) possible. This is due to the inverse-square law, which predicts that the amount of energy dispersed from a single point release of energy (such as a thermonuclear blast) dissipates by the inverse of the square of distance from the single point of release. The result is that the power of a nuclear explosion to rupture hardened structures is greatly decreased by the distance from the impact point of the nuclear weapon. So a near-direct hit is generally necessary, as only diminishing returns are gained by increasing bomb power.
First-strike enabling weapons systems[edit]
Any missile defense system capable of wide-area (e.g., continental) coverage, and especially those enabling destruction of missiles in the boost phase, is a first-strike-enabling weapon because it allows for a nuclear strike to be launched with reduced fear of mutual assured destruction. Such a system has never been deployed, although a limited continental missile defense capability has been deployed by the U.S., but it is capable of defending against only a handful of missiles.
This does not apply, in general, to terminal missile defense systems, such as the former U.S. Safeguard Program or the Russian A-35/A-135 systems. Limited-area terminal missile defense systems, defending such targets as ICBM fields, or C4ISTAR facilities may, in fact, be stabilizing, because they ensure survivable retaliatory capacity, and/or survivable de-escalation capacity.
This also might not apply to a "non-discriminatory" space-based missile defense system, even if it is—actually, precisely because it is—of global reach. Such a system would be designed to destroy all weapons launched by any nation in a ballistic trajectory, negating any nation's capability to launch any strike with ballistic missiles, assuming the system was sufficiently robust to repel attacks from all potential threats, and built to open standards openly agreed upon and adhered to. No such system has yet been seriously proposed.