Much has changed since 1945. After World War II ended and the world saw firsthand what an atomic bomb can do, the race to figure out how it works and enrich plutonium was off. Now 70 years later, the stockpile that the world has accumulated can blow our planet hundreds of times over.
Given that we are in a cold war now with Russia yet again (some people don't think that, but just look at the facts), this is quite concerning. At the same time however, nobody, and we mean nobody wants to start a nuclear war, as it'd be our last war as a species.
The idea behind having such major stockpiles of nukes is for deterrence. Many treaties were signed over the years that forbid countries from actually using them, it's just that you're showing the other guys that you've got what it takes and then some. Rogue nations however, are a different story.