Another thing that must be taken into account when one is discussing conflict is the notion of state warfare. Since the Peace of Westphalia in 1648, the nationstate, that is a sovereign area of people (generally, but not necessarily of the same ethnic background) with a common government and set of governing principles (also known as laws), has been the most common belligerent in conflicts. Even before the formal defining of states at Westphalia, however, proto-nationstates such as tribes, clans, kingdoms, empires, and principalities had been warring for centuries. Each one was quite clearly defined, and often marched into battle with distinguishing marks or banners to distinguish between the different sides. But what does this have to do with deterrence?
Well, quite a lot, actually. Deterrence only works if there is a clearly defined set of people who is being deterred from doing something, presumably attacking. Today, conflicts are nowhere nearly so well-defined as they were when the standard of sovereignty was set at Westphalia or when Europe was divvied up following the Napoleonic Wars at the Congress of Vienna. If one nationstate attacked another, you knew who it was you were attacking, and you knew who it was you were defending.
When was the shift from nationstates as belligerents to non-state actors as belligerents made, though? That question is hard to answer. Many modern wars are waged against terrorist groups or other non-state groups without a defined territory or a defined citizenry.
This makes them impossible to deter, at least with nuclear weapons. Here's where I make the obligatory reference to MAD - Mutually Assured Destruction. In the Cold War, NATO and the USSR stockpiled nuclear weapons and had them ready at a minute's notice to ensure that if the other ever even thought about pressing that red button, they would be bombed to smithereens. Essentially, it boils down to "bomb them before they bomb you." Nuclear deterrence is perhaps the ultimate form of deterrence, because almost nothing can stop a nuclear weapon, and if one country does anything at all to provoke a nuclear-armed state, then that country can expect Hell to rain down upon their heads.
This all changes if one of the belligerents is not a state, though. If a terrorist organization was ever in a position to obtain and launch a nuclear weapon against a country, it could do so with effective impunity. Terrorist organizations do not have territory, and, depending on the delivery method of the warhead, they could make it impossible to mount an effective counterattack. Any retaliation with a nuclear weapon against a terrorist organization would have prohibitively high civilian casualty rates and would draw an unacceptable amount of flak from other countries (and rightly so!). Any non-domestic and non-nuclear retaliation would involve the potential forceful violation of sovereignty (read: invasion) of another state, which would not be received well, either. Furthermore, in the event of an invasion or other form of retaliation, the group responsible for the attack could simply up and move to another location - the beauty of not being constrained to a particular territory.
Deterrence may have worked during the Cold War, in fact, it may have saved millions of lives during the Cold War, but the fact is that it is simply an outdated idea. The concept of having more than a very small strategic reserve of nuclear weapons - if any - is absolutely absurd. It is simply billions of dollars that could be spent on measures to make sure a retaliatory strike is never necessary.
(Note: This is not a true examination of nuclear deterrence strategy. Rather, I just assume that if you want to know about that fascinating topic, you'll read the relevant Wikipedia entries, and then continue to explain why nuclear deterrence won't work in modern conflicts. I might write a post on Cold War-era Nuclear Deterrence in the future, but for the moment, I think it's been pretty well covered.)