There is an inherent momentum in nuclear programmes that makes it harder and harder to stop, or reverse, the advance towards the threshold.
History teaches that Iran will choose nuclear weapons
One of the most vexing questions regarding the Iranian nuclear crisis is whether or not Tehran already intends to build weapons or whether it just wants the option to do so in the future. This is vital in determining the diplomatic margin of manoeuvre that the international community has vis-a-vis Iran.
In both the West and the Middle East, many observers believe that Iran is working towards a "nuclear option" but will stop just short of the nuclear threshold - that is, it will avoid building a nuclear device. This would be, on the face of it, the most rational choice for Tehran: having a "breakout" capability, but avoiding the provocation of testing a nuclear weapon and withdrawal from the Non-Proliferation Treaty.
However, this is a very unlikely outcome. Countries rarely stop a military-orientated nuclear programme below the threshold. When so much investment has been made, it is too tempting to go all the way. India did so for a while: it probably did not build nuclear weapons before the late 1980s, but that was after it had tested a device in 1974, an event that shook the world. Japan is often touted as a possible model for Iran - including by some in Tehran - but it is a totally inappropriate comparison.
The Japanese uranium enrichment programme has sound economic rationales, and Tokyo has not built nuclear-capable ballistic missiles. And there is absolutely no evidence that Japan - the most inspected country in the world because of the size of its nuclear complex - has ever conducted any "weaponisation" study.
Countries that did stop before reaching the threshold were at a far less advanced stage than Iran is today. Sweden, Argentina, South Korea and Libya had military intentions, but they all stopped their programmes, for various reasons, at fairly early stages. The only exceptions to this rule are Iraq and Brazil.
The first one is well-known. Baghdad had a large hidden uranium enrichment programme and was very close to the nuclear threshold when Saddam Hussein invaded Kuwait in 1991. Had Iraq not been defeated by the international coalition, and placed under a United Nations regime of surveillance and sanctions, it would very likely have built nuclear weapons.
The case of Brazil is lesser-known, but provides an interesting example because of its eerie similarity to Iran's programme. In the 1970s and 1980s, Brasilia, then under military rule, conducted two nuclear programmes. The first was civilian and open: in 1987, Brazil proudly announced that it had succeeded in enriching uranium to 20 per cent.
The second one was secret and run by the military. It included a parallel enrichment programme, reprocessing activities, ballistic missiles and weaponisation studies. A test site was even built. That second programme was partly revealed in 1988 by Jose Sarney, Brazil's new civilian president, and then fully exposed and shut down in 1990.
The history of the French nuclear programme can also provide some interesting lessons when trying to fathom the dynamics of Iran's venture. When the French Atomic Energy Commissary was created in 1945, its mission was to explore all dimensions of nuclear science and technology, be it civilian or military.
But the idea of building the bomb rapidly gained support throughout the military. Three rationales appeared in the French debate. One was modernity and the benefits that nuclear technology could bring to the economy and to the armed forces. The second was prestige: France needed to regain its status in Europe after the Second World War.
Finally, atomic weapons were seen as a security guarantee against another invasion, or to ensure that no external power would again be able to coerce France. By 1956, there was so much investment in nuclear infrastructure that any decision to stop would have been financially and politically costly.
Two reactors able to produce weapon-grade plutonium were being built, and a decision was taken to build a reprocessing plant. France was on the verge of becoming a nuclear power, but its intentions were still publicly undeclared.
The lessons from these historical case studies are clear. To ask whether or not a country has actually decided to build the bomb may be pointless. Some countries can arrive at the threshold without ever having made a firm political decision to make nuclear weapons. There is an inherent momentum in nuclear programmes that makes it harder and harder to stop, or reverse, the advance towards the threshold. The bottom line is that Iran is likely to cross the threshold eventually unless there is military action against it, or if regime change comes about.
This calls for a sobering look at the negotiation process. The prospects for a compromise are becoming increasingly unlikely. So what options remain? Recommending a military strike against Iran remains an unappealing option - the last thing the region needs is another war. However, betting on a regime change in Tehran any time soon would also be unreasonable.
The rational course of action is thus to increase pressure on Iran through strengthened sanctions, and perhaps also through the threat of military action in case it is seen as crossing the threshold. The combined effects of sanctions, economic mismanagement and domestic discontent, as well as the fear of a US military strike, may at some point give pause to the Iranian leaders if they fear for the existence of the regime.
Bruno Tertrais is a senior research fellow at the Fondation pour la Recherche Stratégique in France