No countries should have nuclear weapons. Having 2 or more countries with nuclear weapons will always have the threat of a nuclear war, which could end up destroying the world. If only 1 country had nuclear weapons, they would be able to do whatever they wanted because everyone else would be severely underpowered.
Back in the Cold War, the only deterrent from a massive conflict to escape laying into World War 3 were nuclear weapons. After the Soveit Union collapsed, many documents regarding how to overcome the U.S. In a conventional war could be won by eleminating there nuclear warheads with missiles. That was 1950. A document 8 years later said that any conflict would escalate into nuclear retention, so they decided not to move on with other conventional needs, instead investing into submarines as a deterrent.
Not all nations should be allowed to have nuclear weapons because not all nations have a stable a government and good intentions with which to create or put nuclear weapons to use. Only nations that are members of the United Nations (UN) should be allowed to have nuclear weapons because the UN has the power to create policies and restrictions on the nuclear programs of specific nations. Even nations with a stable system of government and Westernized methods of elections should be monitored and prevented from acquiring a dangerous amount of nuclear methods. The more nations have nuclear weapons available, the greater the potential risk of catastrophic world destruction. Ergo, only a select few should possess nuclear weapons.