Nuclear war, or atomic war, is war involving combatants deploying nuclear weapons. The United States is the only nation to have actually used nuclear weapons in battle, having in 1945 dropped one on Hiroshima and another on Nagasaki. However, the term is used mainly to discuss the thus-far hypothetical possibility of widespread use of nuclear weapons in a war involving two or more nuclear-armed nations.
During the Cold War the concept of Mutually Assured Destruction (MAD) held that the large numbers of nuclear missiles held by the United States and its European allies, and Russia would maintain the peace. Such an exchange would have killed many millions of individuals directly and possibly induced a nuclear winter which could have led to the extinction of most of humanity and certainly the collapse of global civilization for decades if not centuries.
During the cold war, civil defense efforts in Russia and China were effective, or at least present, while programs in the United States were notably ineffective and unfunded. This is because for much of this period the United States unilaterally pursued MAD. Under MAD, civil defense was actually consdered destabilizing (bad) because it indicated that one could hope to win the war.
The idea that any nuclear conflict would escalate into massive destruction was a challenge for military strategists. A number of interesting concepts were developed. Early nuclear missile were inaccurate which lead to the concept of counter-city strikes - attacks directly on the enemy population leading to a collapse of the enemy will to fight, although it appears that this was the american interpretation of the Russian stance while the Russian strategy was never clearly anti-population. As missile technology improved the emphasis moved to counter-force strikes - attacking the enemy's means of waging war. This was the predominant doctrine from the late 1960s onwards. Additionally the development of warheads (certainly in the US) moved towards delivering a small explosive force more accurately and with a 'cleaner' blast (fewer long-lasting radio-isotopes). In any conflict therefore, damage would have been initially limited to military targets, there may well have been 'witholds' for targets near civilian areas.
The argument was that the destruction of a city would be a military advantage to the attacked.The enemy had used up weapons and a threat in the destruction while the attacked was relieved of the need to defend the city and still had their entire military potential untouched.
Only if a nuclear conflict was extended into a number of 'spasm' strikes (followed by frenzied negotiations?) would direct strikes against civilians occur as the more accurate weapons would be expended early and if one side was 'losing' the potential for using less accurate submarine launched missiles would occur.
Of course, even "surgical" nuclear strikes against military targets were likely to cause death, destruction, and hardship on scales rarely approached in human history.
Current fears of nuclear war are mainly centred around India (first test May 18, 1974, the "Smiling Buddha" test) and Pakistan (first test May 1998), two nations whose majority religions and histories, as well as a territorial dispute in Kashmir and mutual posession of substantial (though probably numbered in dozens rather than thousands) nuclear arsenals makes many extremely nervous. In the case of Pakistan, their unstable government and the threat of radical Islamists allied to Osama Bin Laden seizing power and thus control over the nuclear arsenal has raised additional fears, compunded by the fact that a senior member of the development program, Sultan Bashiruddin Mahmood, is a strong Taliban sympathizer. Additionally, Iran, Iraq, and North Korea are believed by the United States to be attempting to build nuclear weapons.
However, at the moment rather than full-scale war, nuclear terrorism by non-state organisations could well be more likely, as states posessing nuclear weapons are susceptible to retaliation in kind. Geographically-dispersed and mobile terrorist organisations are not so easy to discourage by the threat of retaliation.