Iran's decision to reopen its uranium enrichment facility at Natanz marks a fateful step further towards a nuclear security crisis that the world and the Middle East, let alone the Iranians, can ill afford. Whether the theocrats in Tehran are indulging in brinkmanship or seriously intend to develop nuclear weapons could soon become academic. Long before Iran acquires fissile material it will have scattered politically combustible material that could ignite in a nasty explosion.

It would appear that what Iran started this week amounts to a pilot project rather than a full-scale resumption of uranium enrichment activity. Nobody quite knows and, given a long history of concealment, no one trusts Iran's account. But either process could eventually lead to weapons development and breaks the moratorium Tehran agreed with Britain, France and Germany - the so-called EU3 - in November 2004. Iran may see this as reasserting its right to build its research capacity and access to technology for peaceful purposes; most of the world regards it as an erratic regime advancing to a dangerous new nuclear threshold.