Sixty years ago today, an atom bomb exploded over Hiroshima, killing 80,000 people, mostly civilians. Tens of thousands more died from radiation sickness in the years that followed. The memory of that day offers a horrific reminder of what is at stake when the nuclear non-proliferation regime comes under attack as it is today.

The direct challenge comes from the nuclear ambitions of Iran and North Korea. Both expose serious shortcomings in the non-proliferation treaty (NPT). It permits non-weapons states to develop nuclear fuel cycles, allowing them legitimately to proceed to the brink of weaponisation. If Iran and North Korea are not made to pull back from the final step, there is a danger of a proliferation "cascade" in north-east Asia and the Middle East. Hence the importance of the final offer from the EU 3 - France, Britain and Germany - delivered to Tehran yesterday.