Pressure builds in Iran nuclear stand-off The decade-long stand-off between Iran and the west over Tehran's nuclear programme has long been an excruciating spectacle to watch. Over the past 10 years, there have been many occasions when politicians and pundits predicted the impasse would be broken - that western military action against Iran was imminent or the Iranians were about to come up with serious concessions. On every occasion, the excitement proved unfounded and diplomacy was given more time. As Iran prepares to meet six world powers next week in Kazakhstan for yet more talks over its nuclear ambitions, few are therefore getting excited. Nobody expects the world's diplomats to leave Almaty on February 26 either with a diplomatic breakthrough with Iran or with a total breakdown in talks. But that does not mean diplomats are relaxed. Anyone who thinks this stand-off has several more years left in it may be misguided. As Cliff Kupchan, a senior analyst at Eurasia Group, puts it: "We've got time, but not a lot of time, before this gets really tricky." There are two fundamental reasons why the Iran story will enter a decisive phase in the next 18 months. First, Iran is getting closer to the point where it might be able to make a sudden dash for a nuclear bomb, without the US and Israel being able to stop it through military action. Ali Khamenei, the Iranian supreme leader, has not yet taken the final decision to construct a nuclear weapon. If he were to make that call now it would probably take Iranian scientists at least two months before they could get their centrifuge machines to enrich enough weapons grade uranium for a nuclear device. Two months is enough time for the US and Israel to discover what the Iranians are doing and destroy the nuclear programme before a weapon can be tested. But one year from now, experts say the situation may look different. Iran announced last week that it was beginning to install more advanced centrifuge at its enrichment plant at Natanz that can enrich uranium at two or three times current speed. The move means that in 2014, Iran may be in a position to complete that covert, final dash for a nuclear weapon in a time much shorter than two months. As Mr Kupchan puts it: "There is an extremely dangerous threshold approaching next year when Iran will achieve very low break out times that Israel and the US would find it hard to live with." The progress of the Iranian programme may worry the US and Israel. But then there is a second factor that makes the next 18 months so decisive - the impact of US and EU energy and banking sanctions on Iran. As they try to gauge how the Iranians are thinking, western diplomats are increasingly confident that the weight of those sanctions will soon force the Iranians to back down. According to European diplomats, the imposition of energy sanctions last year mean Iran has now lost $46bn of oil revenues, a loss equivalent to 8 per cent of its annual gross domestic product. The rial meanwhile has lost 40 per cent of its value in recent times. "The Iranian public's purchasing power is declining all the time," says one diplomat. The regime cannot live with this for much longer. If there is to a breakthrough, when could it happen? Most experts seem to think it will not come for at least another six months. First, the supreme leader will not want to make any concessions before the presidential elections in June. According to western diplomats, his aim is to see Mahmoud Ahmadi-Nejad, the current president, out of power. He does not want to give Mr Ahmadi-Nejad the chance to claim that he personally brokered a nuclear deal with the west. Second, the US and its allies are prepared to be patient. They want to woo Iran towards a deal. But they also believe that the pain of economic sanctions means that time is not on the Iranian regime's side. Third, Israel will not go it alone with pre-emptive military action against Iran as it threatened last year. The Israeli military establishment believes it cannot act without US support. Israel is therefore prepared to give diplomacy a little more time. However, the diplomatic dance cannot go on for ever. Iran is advancing its nuclear technology. The pressure of economic sanctions is mounting. Something has to give.