Right now, a single nuclear weapon small enough to fit in a panel truck is capable of giving as much explosive power as a football field of C4 plastic explosive stacked over 7200 feet high (or equivalently the football field stacked over a mile high of TNT). Given this, one is tempted to ask, “does the world really need nuclear weapons?” Although I can’t answer that question scientifically, I think it is fair for me to describe some pertinent facts as best I know them being a reasonably informed observer.
The origin of nuclear weapons was World War II (WWII) where they were used to stop the Japanese invasion of the entire eastern side of the pacific ocean. Back then it was believed that developing and using the nuclear arms would prevent a land invasion of Japan and save many, many more lives than was lost of both American and Japanese soldiers and citizens. Back then, solders largely shot back and forth at each other based on the conventional war of attrition techniques employed at that time. Door to door fighting and incendiary bombing were seen as the only alternative to using the nukes.
Spies in the US scientific community shared the essential details with the Russians as an attempt to force a balance of power with the US and so within a decade, the Russians developed their own fission weapons. Not too long after this, the US developed thermonuclear weapons which not only use fission devices like those used in WWII but now also utilized fusion energy like that powering the sun resulting in substantially more powerful devices. Since then, England, France and China have also developed and demonstrated their capability to manufacture and use nuclear weapons.
The current international non-proliferation treaty (NPT, which the US has also agreed to) drives the end goal of signatories to strive toward complete elimination of nuclear weapons. Some might say that applies to all weapons including guns and other military armaments. I have even heard some people claim that police don’t even need guns. Technically if everybody agreed to follow established rules, we wouldn’t even need police so in an ideal world that would make sense. Historically, any country having only a weak army and apparent wealth has eventually been taken over by a nearby country with a stronger army so to the extent that this is true, a strong military is important to maintain national sovereignty.
Whether a country needs nuclear weapons to maintain its sovereignty in the modern world is a difficult topic. In fact none of the issues listed above really have scientific solutions per se but the basis for nuclear weapons policy is as I understand it is even more involved. The US has nuclear weapons at multiple strategic locations in Europe in such places as Germany, Italy and Belgium (as well as Turkey). By doing this, these countries have the assurance that should Russia or the middle east attack them, they are protected by US nuclear weapons. This allows those countries the cost savings of not having to develop their own nuclear weapons program and so it can be argued to support the NPT to the extent that there are now fewer countries in the world having their own nuclear weapons programs. Given what occurred in WWII, keeping Germany and Italy from seeing any need for a nuclear armed military has some aesthetic attractiveness to should they ever desire world domination again. The US also demonstrates a great deal of protectiveness towards foreign democracies such as Israel and Taiwan so that the State Department can leverage military strength as an implied threat against enemies of those countries.
Many argue that protecting the European countries with our nuclear weapons does not really prevent them from developing their own nuclear weapons as they claim they would neither want nor afford such programs were they removed. Given that India, Pakistan and now apparently North Korea have all created a nuclear weapons program with Iran bringing one online suggests that being the big kid on the block is a reasonably expected stance many countries will continue to seek in the future.
I cannot answer the question as to whether nuclear weapons are really needed. I believe I do understand why we have them now and can reasonably expect them to be around for many years to come based on current US policy. Irrespective of your view of US nuclear weapons, hopefully you now have a more informed perspective.