During this recent Atlantic Council event, the FT’s Demetri Sevastopulo recounted this anecdote concerning authority to launch British nuclear weapons:

a former British defence secretary asked me during the Trump administration,would it be possible for President Trump to launch a nuclear weapon without anyone stopping him, and how did theprocess work. And I said, there are lots of theories; I don’t know the exact answer. How did it work in the U.K? He said, I don’t know. They never briefed me on what I would have to do.

In his last book, President Obama recounted his early exchange of letters with Iran’s Supreme Leader Khamenei:

For all these reasons, my team and I had spent much of the transition discussing how to prevent Iran from getting a nuclear weapon—ideally through diplomacy rather than by starting another war. We settled on a two-step strategy. Because there had been almost no high-level contact between the United States and Iran since 1980, step one involved direct outreach. As I’d said in my inaugural address, we were ready to extend a hand to those willing to unclench their fists. Within weeks of taking office, I’d sent a secret letter to Ayatollah Khamenei through a channel we had with Iranian diplomats at the United Nations, suggesting that we open a dialogue between our two countries on a range of issues, including Iran’s nuclear program. Khamenei’s answer was blunt: Iran had no interest in direct talks. He did, however, take the opportunity to suggest ways the United States could stop being an imperialist bully.

“Guess he’s not unclenching his fist anytime soon,” Rahm said after reading a copy of Khamenei’s letter, which had been translated from Farsi.”

“Only enough to give me the middle finger,” I said.”

I noticed that the AEOI still has Iran’s Policy on Nuclear Non-proliferation and Disarmament on its website:

A- Statement of the Founder of the Islamic Republic of Iran

After the triumph of the Islamic Revolution, the Islamic Government carried out a review on all bilateral, multilateral contracts and treaties. The founder of the Islamic Republic of Iran, the late Imam Khomeini, on a number of various occasions condemned nuclear weapons. In 80s, during public visit, addressing the audience and the world public at large, he said: “… if they continue to make huge atomic weapons and so forth, the world may be pushed into destruction and the major loss will afflict the nations. Everybody, wherever he is, the writers, intellectuals and scholars and scientists throughout the world should enlighten the people of this danger, so that the masses of people will standup vis-à-vis these two powers themselves and prevent the proliferation of these arms.”

B- Statement of the Supreme Leader of the Islamic Republic of Iran

The Islamic Republic of Iran regards use of nuclear and chemical weapons as a cardinal and unforgivable sin. We raised the slogan “Middle East free from nuclear weapons”, and we remain committed to this slogan. This slogan does not mean giving up our right to peaceful uses of nuclear energy and production of nuclear fuel. Peaceful use of nuclear energy is the right of all nations in accordance with rules of international law. All nations should be able to use this clean energy for vital uses in the country and for the people. They need not be dependent on others in exercising this right. Certain western countries that unlawfully possess nuclear weapons want to keep the monopoly of nuclear fuel production for themselves. A surreptitious movement is evolving these days to create a permanent center with an international name and appearance, but in fact in the permanent hands of few western countries, for the monopoly of production and sale of nuclear fuel.

Islamic Republic of Iran has never sought nuclear weapons and will not surrender the right of its people to peaceful use of nuclear energy. Our motto is “Nuclear energy for all, nuclear weapons for none”. We will insist on this motto and know that breaking the monopoly of few western countries in production of nuclear energy in the framework of NPT is good for all sovereign nations, including member states of the Non-Aligned Movement.

Here’s a short presentation about the UN Secretary-General’s Mechanism (UNSGM) for Investigation of Alleged Use of Chemical, Biological and Toxin Weapons

According to this MOD press release, Defense Minister Raksha Mantri Shri Rajnath Singh told his Russian counterpart Sergei Shoigu that

the nuclear option should not be resorted to by any side as the prospect of the usage of nuclear or radiological weapons goes against the basic tenets of humanity.

A few days back, Iranian Supreme Leader Khamenei explained Iran’s development and acquisition of nuclear technology:

Fundamental achievements in the nuclear industry, the issue of nuclear technology is a long story. There is a lot to be said in this regard, but our time is limited now. We have gained access to fundamental issues in nuclear industry. We are not seeking weapons or bombs or things like that, but this industry itself has many benefits. We were able to gain access [to it].

The first producer of nuclear technology was Germany during Hitler’s time. The Americans then took this technology from them and some other countries were able to take it from the Americans through different kinds of scientific theft. And then those countries gave the technology to other like-minded countries, and so on. So, that is how they have gained access to it.

Not us! Of course, the very first insufficient groundwork that was laid out was from abroad and it was very insignificant. But the actual work that was done was carried out by Iranian elites and they did it with power and they did it with hope

This 2001 paper by Richard Garwin serves as a nice reference for basic nuclear weapons facts. For example,

U.S. nuclear weapons (and perhaps those of
the other four Nuclear Weapon States under the 1970
Non-Proliferation Treaty, NPT) now consist of hollow
plutonium shells surrounded by high explosive. The shell
can be accelerated and then abruptly arrests itself by
symmetry, leading to greater compression than can be
achieved in a solid sphere. Accordingly, the Pu content in
a U.S. nuclear weapon is as little as 4 kg.

India recently tested an SLBM:

INS Arihant carried out a successful launch of a Submarine Launched Ballistic Missile (SLBM) on October 14, 2022. The missile was tested to a predetermined range and impacted the target area in the Bay of Bengal with very high accuracy. All operational and technological parameters of the weapon system have been validated.

The successful user training launch of the SLBM by INS Arihant is significant to prove crew competency and validate the SSBN programme, a key element of India’s nuclear deterrence capability. A robust, survivable and assured retaliatory capability is in keeping with India’s policy to have ‘Credible Minimum Deterrence‘ that underpins its ‘No First Use’ commitment.

I’m sure someone’s mentioned it, but XX published this FR notice in 2015 which notes that the End-User Review Committee

determined that the National University of Defense Technology (NUDT), the National Supercomputing Center in Changsha (NSCC-CS), National Supercomputing Center in Guangzhou (NSCC-GZ), and the National Supercomputing Center in Tianjin (NSCC-TJ), all located in the People’s Republic of China, meet the guidelines listed under § 744.11(b): Entities for which there is reasonable cause to believe, based on specific and articulated facts, that an entity has been involved, is involved, or poses a significant risk of being or becoming involved in activities that are contrary to the national security or foreign policy interests of the United States and those acting on behalf of such entities may be added to the Entity List pursuant to this section. Specifically, NUDT has used U.S.-origin multicores, boards, and (co)processors to produce the TianHe-1A and TianHe-2 supercomputers located at the National Supercomputing Centers in Changsha, Guangzhou, and Tianjin. The TianHe-1A and TianHe-2 supercomputers are believed to be used in nuclear explosive activities as described in § 744.2(a) of the EAR.

In this report, titled Report No. 27 (2007–2008) to the Storting Disarmament and Non-proliferation, the Norwegian Foreign Ministry stated that

The future of the 1987 Intermediate-range Nuclear Forces Treaty (INF), which prohibits the US and Russia from possessing ground-launched ballistic and cruise missiles with ranges between 500 and 5500 kilometres, is also uncertain, because Russia has expressed doubt as to whether the treaty still serves the country’s security interests.