NORTH KOREA’S third nuclear test on February 12th (and its first for nearly four years) comes just two months after it launched a “weather observation” satellite into orbit. Both were in defiance of UN sanctions against the pariah state’s nuclear programme. The significance of this most recent underground test is that it is claimed to involve a powerful miniaturised device that could be small enough to attach to a missile.

North Korea is now virtually alone in conducting nuclear-weapons tests, as the chart below shows. Pakistan and India last carried out tests in 1998, while France and China conducted their last underground tests in 1996 just before signing the Comprehensive Test Ban Treaty (CTBT). Although the CTBT has yet to come into effect because several states (America, China, Egypt, Iran and Israel) have signed but not ratified it, while India, North Korea and Pakistan have not yet signed it, the treaty has exerted a degree of moral force.

Experts disagree over the usefulness of testing. Technologically sophisticated nuclear states that have collected data from tests in the past appear able to rely on computerised simulations to ensure the continued effectiveness of their nuclear arsenals. But some American critics of the treaty contend that at some point it may be necessary to carry out tests again to ensure the safety of ageing weapons, most of which are now well over 20 years old. However, for North Korea, as it has been for other emerging nuclear weapons states, testing is above all a political act, a declaration of technical proficiency and military power. The world seems to have decided on a policy of uneasy containment and isolation as far as a nuclear-armed North Korea is concerned. That is unlikely to be the response if Iran, which has shared missile and nuclear technology with North Korea for years, becomes the next country to test a device. In Iran’s case, a test alone would be regarded as an act of war by Israel and, probably, America too.