Personally I'm very happy nuclear weapons have been developed, yes they have killed hundreds of thousands directly and many more indirectly, but the sheer power if harnessed I think will be the future of human development at least on the technological front. Starting from the beginning, everyone was developing nuclear weapons; America was hosting most of the allied development as far as I'm aware, while Germany and Japan both had their programs. The Americans simply had vastly more resources so got there first.
Howeve sincle World War II thier affect has beenso great it's difficult to calculate, on the one hand the world is perhaps more unstable now thanks to the proliferation of them, but once the Russians aquired them from a defector, it made the cost of large sclae wars to high even for them to attempt it. After all, World War II itself killed millions, if the Cold war had directly sparked off (which it may well have done given the ideological reasons were far from new), millions more would have died, and maybe have stalled international development in it's tracks, and the world would ahve continued scrapping over land resources and ideals for a long time to come. It's my opinion that nuclear weapons are one of the key things to have (potentially) broken the cycle of conflict leading to conflcit, simply because victory in nuclear war is almost impossible against even just a slightly equal opponent.
This itself does fo course pose it's own problems, as for instance the US national missile defence system has shown; America and maybe some of it's allies will be protected from the prevailent nuclear capability, allowing it to use more precise nuclear weapons with near impunity as has been shown by US plans to increase their use of tactical nuclear weapons for bunker busting. But even so public opinion in the US hasn't kept up with the total power philosophy popular with the government, so it is likely to be restrained at least to some extent.
The real value of nuclear technology to my mind is not in fighting each other though or preventing it; nuclear power is the only long term energy solution for the developed world. environmentalist groups will happily plug solar, wave, wind power etc but it is simply impossible for these sytems to reliably power whole countries, leaving either fossil fuels whch are in incredibly short supply, and nuclear power. The latter is readily available given the efficiency in terms of material usage of the fuel supply, and the potential for further development (fusion technology for example) could provide a very long term proposition which could keep up with the pace fo development for centuries, assuming of course we don't all revert to spiky sticks and living in tree houses before then

. Hmm, looking back at that I think I'm getting a bit far fetched, I'd better stop

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