Since the early 1990s, the maximum violence in acts of international terrorism has steadily increased. It is on the basis of this trajectory - and because of al-Qaeda's stated aims to acquire and use a so-called 'weapon of mass destruction' - that many analysts have concluded that a nuclear terror attack is likely only a matter of time. Others argue that this is alarmism, but the fissile material is available, or could become available, from a Russian nuclear stockpile that remains dangerously insecure. An improvised nuclear device would be difficult, but not too difficult, to build. And there are terrorist groups that have already demonstrated the technical abilities and organisational reach to make us very worried.