Nuclear weapons were invented in the United States, The original motivation for developing a nuclear weapon was to beat the Nazis to the bomb, but Hitler had been too occupied with fighting off conventional attacks, especially toward the end of the war, and never put enough priority on developing a nuclear weapon. Germany certainly had the talent to run its own Manhattan Project. Germany was defeated before it was in a position to use a nuclear weapon on London or New York.
Though the fire-bombing of Dresden was dreadful, the US never used a nuclear weapon on a German city. The US did use a nuclear weapon to bomb Japan into unconditional surrender.
Since the WWII bombing of Nagasaki, many nations developed their own nuclear weapons. The US, Britain, France, India, Pakistan, North Korea each have a nuclear arsenal, and Israel, Japan, Iran and Saudi Arabia certainly have the capability to stock an arsenal.
Nuclear weapon technology is pretty well understood now, so any nation willing to spend the money could have their very own nuclear weapon. So-far, since WWII, no nation has used their nuclear weapons on another nation. Sure, there have been many times when the thought has crossed the minds of national leaders. A nuke might have stopped the advance of China during the Korean War. Israel might have nuked Egypt when things looked bad during the Yom Kippur war. India and Pakistan have threatened to nuke each other, over Kashmir or terror attacks. Iran and Israel have traded threats.
Nuclear weapons have proliferated, in spite of numerous resolutions and treaties, because of the perceived need to threaten other nuclear-capable countries with retaliation. Every nation with a nuclear arsenal claims that the purpose of the arsenal is only to deter a nuclear attack (or an overwhelming conventional attack) from another nation. As each nation joins the nuclear club, it solemnly states that the weapon is for deterrence only, but perhaps some of the motivation might be macho prestige.
No nation, including US, Iran and North Korea, admits having plans to use their arsenal to start a war, to subdue another nation.
There is some doubt about the intentions of Iran, and this is the reason for the long term negotiations to ensure that Iran never develops a nuclear weapon.
Now that Islamic terrorists have become the big enemy, it seems likely that if a terror group could acquire a nuclear weapon, they would actually use it. A group which sends out suicide bombers might not fear nuclear retaliation.
So-far, there have been no negotiations with Al Qaeda, ISIS or any other terror group, to gain assurance that their terror attacks will remain non-nuclear.
If an "Islamic Bomb" actually destroyed a US city, in a terror attack far worse than 9/11, the knee-jerk reaction might be to throw a nuclear weapon at Mecca, especially if it were not clear which Muslim country had provided the bomb. The target list of the Tridents is sure to include the coordinates of Mecca.
The US has deployed a major nuclear deterrent in its fleet of Trident Submarines, hidden under water all over the world, ready to deliver H-bombs from many directions at once. The threat of the Tridents could well be the reason why nobody, not even a terrorist group, has so-far launched a nuclear weapon. I suppose enough tests have been run to convince the military establishments of all countries and terror groups that the Trident capability is real and the submarines might well be used by the US to launch nuclear obliteration on anyone who uses their own nuclear weapon. While it's nice to have nuclear weapons for prestige, it's not safe or effective to use them for an aggressive war.
And besides, any nuclear retaliation is constrained by fear of setting off a chain-reaction multi-nation nuclear exchange, which could kill the planet we all share. Would the UN or any coalition be able to get an armlock on a nuclear-armed terrorist group quickly enough to prevent cycles of retaliation? A nuclear wipeout of Mecca sure would make good Muslims feel bad, but it's not likely to bother Al Qaeda, or any other group that sees nuclear armageddon as a quick way to bring on the Mahdi.
So, if nuclear weapons are only good for prestige and retaliatory threat, and everyone is sure that they could never launch a nuclear attack requiring retaliation, then why can't we redistribute the weapons so that everyone has an arsenal? The US alone could supply enough nuclear weapons to put every nation into the club. So, if everyone had the bomb, would this be nuclear stalemate and we could all relax?
Of course the problem is still the "madman," who would mindlessly use a nuclear weapon to destroy his enemy, even if it meant personal or even planetary suicide. The only protection against a nuclear madman is to make the process of pushing the button too complicated and involve too many people. Trident submarine crews follow such a protocol.
Some religious leader might motivate an entire launch crew to fire their weapon, but it doesn't seem likely. Political agreement to launch a first-strike nuclear attack seems extremely unlikely, whether in a democracy, theocracy or an authoritarian state. Even in a large decentralized group like Al Qaeda, word of a plan for catastrophic attack would leak out. One hopes that the experience of the 9/11 disaster has made it less likely that such leaks will be ignored.
There remains the grim possibility of some crazy person, beyone the control of a government, stealing a nuclear weapon and being able to launch it. This possibility is minimized by good physical control of everyone's arsenal.
Nuclear non-proliferation has failed; we may as well change the name to nuclear proliferation -- everybody wants their own. Strategic arms limitation hasn't worked either. Even if some weapons have been deactivated, the military-industrial complex pushes for modernization of the rest.
The one thing that has worked is nuclear deterrence. Since 1945, the world has seen the Korean War, the Vietnam War, the Israel-Arab wars, the Gulf War and the current wars in Syria, Afghanistan and Iraq, just to name some of those all of which the nuclear-armed US has been involved in, at least indirectly. General MacArthur wanted to use nuclear weapons in Korea. General LeMay wanted to use them on Cuba. Russia probably would have liked to use them on the Chechens. Pakistan and India have both developed nuclear weapons, but (so-far) have not used them. Israel considered using its (officially non-existent) nuclear weapons on Egypt when things were going badly in 1973. Saddam Hussein threatened use of weapons of mass destruction; maybe he was madman enough to have used them if Iraq had really possessed them.
Maybe all those pictures of mushroom clouds from the nuclear tests over the years were especially frightening. Maybe measurements of nuclear fallout, far from the site of the test, gave a hint of how sick we could make the world if there were a real nuclear exchange.
Maybe it has been clear for a long time that nobody is going to win a nuclear war. President Reagan said he wanted to make nuclear weapons impotent and obsolete. Well, given the current nuclear proliferation and strategic stalemate, nuclear weapons sure do look impotent now. But nuclear weapons are not obsolete if we can't get rid of them.
Maybe instead of negotiating to deny Iran the right to develop nuclear weapons, we should negotiate to decomission the existing nuclear arsenals of every country that has one. Deny everyone a national nuke, not just Iran.
We can use the recycled Uranium and Plutonium to mass produce compact nuclear reactors which can supply abundant power for all the world, rather than continue holding the threat of destruction over everyone.
Would Iran like this idea?