What it is: During the Cold War, a surprise ('pre-emptive') attack with a nuclear missile, intended to destroy most of the opponent's nuclear missiles before they could be used.
What it means: Pre-emption in nuclear warfare is highly problematic. It persuades opposing sides that they must all be in a position to attack first, in order to prevent them being attacked first. This means that all sides aim to increase their firepower to a point ahead of the others - which of course leads to spiralling development and manufacture of yet more weapons. The risk of nuclear disaster therefore grows too. Pre-emption is tricky whatever the form of warfare. Most countries justify keeping armies and supplies of weapons by saying they are for 'defence in the case of attack'. If such countries make pre-emptive strikes, that justification is shown to be a lie. (And if full-scale war follows, the attacker is in no position to complain.) Even a pre-emptive strike made because an attack is confidently predicted is more likely to cause war than stall it. In any case, prediction can never be rock-solidly reliable, and makes the future even more unstable. The history of the attack on Iraq in 2003, and its aftermath, is a disturbing example of the dangers of pre-emption. Pre-emption (for whatever reason) is also an act of aggression - which is illegal under international laws of war.
Think about it: Trying to wipe out an opponent's nuclear arsenal before it's used may seem like a sound manoeuvre, at least to military logicians - but suppose the opponent has the same idea? Either way, it's a recipe for wholesale destruction. On the other hand, if people can really start to think that disarming opponents is better than fighting it out, then nobody need have weapons at all. Or so logic suggests. How might people be persuaded that hitting back isn't the only way to respond to an opponent? How might opponents be persuaded that there are less dangerous methods than war - during which 'laws of war' are forgotten?