Simply put, we're entering an age where nuclear weapons are no longer the private stock of major powers.
For instance, if, 20 years hence, a nuclear bomb goes off in our country, and we find that it was of Pakistani design, that doesn't mean Pakistan is the culprit. They may have helped Saudi Arabia build their program, and the Saudis may have helped Venezuala, who sold their weapons to Nigeria for cash, who then lost their weapons during a *rebel attack*, and lo and behold, it looks like al-Qa'ida got their hands on a weapon.
Who do we bomb, Mr. President?
These nations aren't stupid. Back in the old days, the Soviets had no way to launder their weapons into third party hands. The nuclear club was small, and the members were known to each other intimately. Now, the real danger of proliferation is not that the weapons are more readily available. It's that they're becoming a commodity.
If Iraq attacks Kuwait using Soviet tanks, well, that's just business. No one blamed the Russians for what the purchaser did with the weapons. While we're not quite there yet, proliferation will put nuclear weapons, by virtue of their commonality, into the same category as main battle tanks. Until then, some level of plausible deniability, like what I stated above, will have to be observed. But soon enough, so many nations will have nuclear weapons, the unique political status they held will be gone.
Welcome to the 21st century.
Ping.