Iran
Kenneth Waltz wrote a provocative essay (pay-walled) for Foreign Affairs arguing that an Iranian nuclear weapon would be a good thing (or not as bad a thing as most assume). In an interview with the Diplomat, he fleshes out his thoughts, comparing the development with Pakistan's nuclear breakout:
'India quite naturally did not want Pakistan to become a nuclear state. A second nuclear state cramps the style of the first. It is hard to imagine one nuclear state acquiescing easily or gracefully to its adversary going nuclear. But certainly in the long run, the nuclear weapons have meant peace on the subcontinent. This is in GREAT contrast to the expectations that most people entertained. Statements abounded by pundits, academics, journalists that suggested that nuclear weapons would mean war on the subcontinent. These experts all denied that the nuclear relationship between India and Pakistan could be like that between the U.S. and the Soviet Union. When two countries have nuclear weapons it becomes impossible for either to strike at the manifestly vital interests of the other. It remains very possible, however, for nuclear states to engage in skirmishes, and those can of course be deadly. A historical example is the Soviet-China border disputes (1969), and a more recent one is the Mumbai attacks. But never have any of these skirmishes gotten so out of hand as to escalate to full-scale war.'
The comparison with Pakistan is interesting. Pakistan, like Iran, is a state that nurtures alliances with terrorist groups yet never once passed off a bomb to one of these groups. Pakistan is an Islamic state yet never embraced national suicide by attacking their arch-enemy India. So the fact that the U.S and India have thus far lived, albeit very uncomfortably, with a nuclear Pakistan is proof that it could do so with a nuclear Iran. The India-Pakistan rivalry is orders of magnitude more intense than anything between Iran and Israel, and it has not devolved into a nuclear Armageddon.
On the flip side, the Pakistani example also shows why Waltz is being a bit too sanguine. Pakistan's nuclear weapons are source of huge insecurity both for Pakistan's neighbors and for the world - less because of fears that the Pakistani military will launch them, but that the Pakistani state will break down and the military would lose custody of one or more weapons. While a complete state collapse doesn't appear to be a near-term possibility, the country is far from stable. The fact that it has nuclear weapons is an added degree of international heartburn.
Iran doesn't have as much internal instability as Pakistan (a fact which some U.S. lawmakers apparently want to remedy by funding an anti-Iranian regime terrorist group) but it has been challenged recently. Once Iran acquires nuclear weapons (if it ultimately does so), internal instability becomes that much more dangerous. In Waltz' view, the spread of nuclear weapons stabilizes state-to-state relations, but there's the pressing problem of what happens if those nuclear weapons states break down.