To what extent can the Nuclear Non- Proliferation Treaty of 1968 be seen as a success?
This treaty cannot be seen as a very significant success. There are three main reasons for this.
First, most of the countries that were likely to want nuclear weapons already had them before the treaty was signed. The fact that the five major powers already had such weapons meant that the treaty was something that went into effect only after most countries that seriously wanted the weapons already had them.
Second, there has been very little movement towards disarmament. This was supposed to be a part of the treaty and yet the five nuclear powers have done little to actually disarm.
Finally, the treaty has not been very successful in preventing countries from getting and keeping nuclear weapons. India, Pakistan, North Korea and (presumably) Israel all have developed nuclear weapons since the treaty was put in place. Iran is moving towards it. A lack of cooperation from countries like Russia and China makes it very hard to put any teeth in the treaty. Instead, any enforcement of the treaty has to be done by a few countries, sometimes (as with Israel's attack on Iraq in 1981) through unilateral violence.
Thus, the treaty cannot be seen as much of a success.