The US/UK started investigation in the late 30's to find out more about nuclear weapons, and on a certain point the Manhattan project was born in '42. And succesfully tested in '45. So no, it wasn't, like i said, even without the war there might've been a point where WMD would be created, for defensive purposes only or propaganda, a feasible thing.

The US had it's rivalry with the Soviet union, which existed before Nazi germany, but that was a tension that involved military/espionage. Countries find more need to face eachother when it's vital or not to appear too weak, or to gain the upperhand with military or the world economy, there are many reasons for such cases of rivalry. But at this moment, i believe the US doesn't need to have skirmishes with Japan, which has been the leader of breakthroughs for some time now. Like you said for example in cybernetics. Japan today poses no danger to anyone, thus the US doesnt need to act as the big rival to them. When i look at the world today, i see more and more of Russia's older tendencies returning, like the time when Putin ordered bombers back on the same routes/patrols during the Cold war Era, or threatening to point Missiles at europe. Remember the US plans for a defensive shield near Russia?

As for any outside intelligent species, stop hoping for them to be revealed for quite some time if they exist, either for selfish reasons or because they think the world isn't ready for disclosure. Now these beings could choose for themselves to let the world know they exist, but that is a whole different subject.

And yes, there are alot of countries that have tested Nuclear weapons, and as for Nuclear powers on the world there are still enough to eradicate the globe many times over. Russia/US/Pakistan/Israel/India/North Korea(?)/UK/France and China, they all have nuclear weapons and some smaller tried to obtain them, South Africa and Iran(I have doubts). Former Soviet nations have returned these weapons to Russia of today. Best hope these weapons wont be used in an all-out war, because there wont be a bit left of this world that you would recognize.

The biggest danger with nuclear weapons in these days is nuclear terrorism, it takes just one to blow up and as a result martial law, mass panic and retaliation following. The US blaming Iran for that attack and as a result of thát shaking the world economy, import of oil, breaking lines with Iran's trading partners in the end. (Russia/China i believe) Next to that, just imagine the shock in everyone's mind and the disbelief, it would be 9/11 all over but a thousand times worse.

It's like a domino effect. And it's another step to the feared scenario of WW3. (I keep an eye on such things)