Do Nuclear Weapons Deter?
Suppose that Iran got a nuclear bomb. Would it be of any use to it? Let's say the US attacks Iran and kills half of its population. Iran decides to retaliate by nuking Israel and destroying half of its population. What happens next? Israel responds with a nuclear strike eliminating the rest of Iran. Hence the Iranian strike was both immoral (for millions of innocent Israelis are murdered) and irrational, because not only did the Iranian government get blood on its hands, but the entire Iran was wiped out rather than merely 1/2 of it.
Similarly, suppose that during the Cold War the Russians nuked the New York City. Would it be reasonable for Americans to retaliate, wiping out Moscow? First, it would clearly be immoral: to the death toll of 8 million innocent New Yorkers the US would add 5 million (or however many) innocent Muscovites. Second, it would invite further attack on the part of the Russians who would after the destruction of Moscow annihilate, say, Chicago. If Americans replied by laying waste to Leningrad, the Russians would get madder still and obliterate Los Angeles. And so it would go until the entire human civilization would be gone. The purpose of war, which has traditionally been to conquer and rule, is defeated. If all is destroyed, what's there left to rule? Empty ruins? Suppose further that the two countries exchanging nuclear blows are geographically next to each other. Then the secondary effects of a nuclear attack (radiation, etc.) by any country will affect that country, as well. That is an additional reason for a victimized nation not to act in response to a nuclear attack by a neighbor state.
Proceeding from the next-to-last strike, it is clear that any retaliation makes neither moral nor self-interested sense. Hence, if these simple considerations are true, the US can attack Iran with nuclear weapons and be fairly sure that there will be no retaliation. Similarly, Iran can nuke Israel and hold, correctly, that the latter is unlikely to respond in kind.
It follows that nukes do not deter first nuclear strikes. Do they deter attacks with conventional weapons? At first glance it seems unlikely, because nukes are so powerful that they cannot be pinpointed against the attacker on a battleground and will harm their possessor, as well, when detonated. But their deterrent value could still lie in the threat to destroy the cities and the economy of the offender country that uses conventional weapons. Suppose that the Lebanese had got the bomb. Israel could no longer treat them like dirt, for they could in principle threaten to blow up Jerusalem, for which act, as has been suggested, it would make no sense for Israel to retaliate in kind.
It is true that such a response by Lebanon would be disproportionate if triggered and therefore morally wrong, but it would serve to deter aggression nonetheless. To avoid misunderstanding, I am arguing that conventional but not nuclear attacks can be deterred by a nation's becoming nuclear. The reason is that the former kind of attack aims at control, while the latter, at pure destruction. To destroy as a response to a destructive act makes no sense, especially if there is any kind of trade going on between the belligerents, and will to boot snowball out of control. But wiping out the enemy's women and children will be a sufficient threat to prevent any state from challenging the existing regime of a foreign nation through non-nuclear means.
The question then, is this deterring function a sufficient good such that we should encourage every country to get WMDs? Instead of fighting with Iran over its (non-existent) nukes, should we subsidize their building? (Or, better, should we simply give them some of our missiles?) I say no, because of the collectivist nature of nuclear attacks. Instead of seeing the world in terms of governments fighting for supremacy, we should adopt radical individualism and, as its ultimate manifestation, market anarchism, under which no private entity (for there will be no governments as such) will be permitted to own WMDs, and such possession will be treated as a tort or a crime.
So, nukes can deter a foreign government from trying to overthrow the government of another country, such as the US overthrowing the Iranian state. But under anarcho-capitalism the ownership of such weapons will not only be illegal but pointless, as well. Will the city of Kent, Ohio need them to protect itself against the encroachments of the city of Akron, Ohio? If it weren't for the federal government and its massive stock of WMDs, would we all be speaking Akronese now? But then the only purpose of nuclear weapons, viz., to deter conventional attacks, is voided under anarchism or even a regime in which the biggest government is that of a city-state. As preparation for this glorious state of affairs, achieved in part through abolition of the US federal government, there should be a unilateral disarmament of all WMDs.