The Nonproliferation treaty (NPT) was opened for signing in 1968. the Treaty entered into force in 1970.
Three states—India, Israel, and Pakistan—never joined the NPT and are known to possess nuclear weapons. Claiming its nuclear program was for peaceful purposes, India first tested a nuclear explosive device in 1974. That test spurred Pakistan to ramp up work on its secret nuclear weapons program. India and Pakistan both publicly demonstrated their nuclear weapon capabilities with a round of tit-for-tat nuclear tests in May 1998.
North Korea went nuclear in 2009. They had a dud test in 2006 but a successful test in 2009.
South Korea has been offered shared nuclear weapons by the USA but has refused them. South Korea has the clear conventional weapons advantage since the 1990s. Seoul has responded to North Korea’s growing nuclear capabilities in two ways. First, it has sought greater reassurances from the United States about its extended deterrence policies in the Asia-Pacific. Second, it is pursuing the necessary military capabilities—such as ISR, precision-strike, and missile defense—to preemptively destroy North Korea’s nuclear arsenal under the doctrine of “active deterrence.”
Main concern is proliferation after Iran goes nuclear
The main concern is that there will be nuclear proliferation after Iran goes nuclear. The belief is that Saudi Arabia, Egypt, Jordan and Turkey would pursue nuclear weapons.
Sanctions and covert action have been the primary means of deterring states from pursuing nuclear programs.
Can other means be used to deter nuclear proliferation ?
Nuclear Weapons Sharing
It is believed that Saudi Arabia might buy nuclear weapons from Pakistan or there already is an agreement. Pakistan is poor but has 100 nuclear weapons and may be building 10-20 more each year.
Officials in the West believe Saudi Arabia and Pakistan have an understanding in which Islamabad would supply the kingdom with warheads if security in the Persian Gulf was threatened. A Western official told The Times that Riyadh could have the nuclear warheads in a matter of days of approaching Islamabad. Pakistan's ambassador to Saudi Arabia, Mohammed Naeem Khan was quoted as saying that "Pakistan considers the security of Saudi Arabia not just as a diplomatic or an internal matter but as a personal matter." Naeem also said that the Saudi leadership considered Pakistan and Saudi Arabia to be one country.
Belgium, Germany, Italy, the Netherlands and Turkey are still hosting U.S. nuclear weapons as part of NATO's nuclear sharing policy. Canada hosted weapons until 1984, and Greece until 2001. The United Kingdom also received U.S. tactical nuclear weapons such as nuclear artillery and Lance missiles until 1992, despite the UK being a nuclear weapons state in its own right; these were mainly deployed in Germany.
The number of shared nuclear weapons
Saudi Arabia and Turkey already have arrangements to match early levels of a nuclear Iran.
It is believed that Iran wants nuclear weapons to gain more regional military dominance.
A robust program that would be triggered if Iran went nuclear would be to support Egypt, Jordan, Israel, Saudi Arabia and Turkey with strengthened conventional capabilities and more nuclear weapons sharing to a greater level than Iran achieves would prevent Iran from achieving any regional dominance.
Nuclear weapons sharing from the USA would provide more control than allowing more independent nuclear programs.
If such a program was signaled as being the official response, it could be part of the program to disincentivize Iran from going for nuclear bombs.
SOURCEs - Wikipedia