The other historical aspect of things like nuclear weapons is "leakage", the idea that when one country develops nuclear weapons other advanced nations aren't going to be far behind, partly because they can learn quickly from what their competitors do. Ideally, in many areas of research it would be harder to sustain a large technological advantage in the modern era because information spreads much faster.
The limiting factor in the Modern/Contemporary Eras is not so much the basic research as it is building the Infrastructure required to actually produce new devices/weapons. Last I looked (several years ago) there were an estimated 20 or more countries in the world that had the technical knowledge to build nuclear devices or weapons, but for most of them it would take 3 - 8 years to develop and construct the facilities needed to make the parts needed to build a bomb.
Even without the 'diplomatic factor' that makes acquiring nuclear weapons problematical (Germany has had the technical knowledge for decades, but 'firestorm' doesn't begin to describe the diplomatic consequences of Germany announcing that it was going to build nuclear weapons!) this factor applies in many other areas of advanced weaponry. My two favorite historical examples are Battleships and Medium Tanks.
The Battleship was the symbol of First Class Naval Power all over the world from 1906 to the 1940s, yet out of over 50 countries in the world, only 8 - Britain, Germany, France, Austria, Russia, Japan, Italy, and the USA - ever actually built a Battleship, and those countries built all of their battleships and battlecruisers in a total of 56 shipyards (of which 24 were in Great Britain or the USA alone) - the industrial requirements to build 20 - 70,000 ton ships, big guns, and armor plate by the 1000s of tons was simply prohibitive for most of the world's economies.
The medium tank (21 tons or more with a cannon armament) became the symbol of military prowess in World War Two, but again, out of over 50 countries in the world, by 1945 only 7 - Britain, Germany, USA. Russia, France, Italy, Japan (and the last two just barely) had designed and manufactured medium tanks in any quantity at all. Again the industrial requirements for automotive, armor plate, and artillery manufacturing that went into a medium tank were beyond most of the world. Even in the 21st century. Main Battle Tanks have only been built by 11 out of 200+ nations: USA, Russia, Britain, Germany, France, Brazil, China, Japan, India, Israel and Sweden. On the other hand, and not modeled in Civ at all, main battle tanks, medium tanks, and battleships were all either built for other countries by those that could build them, or sold to them in quantity for various commercial or diplomatic reasons.