Gordon Brown's three little words about replacing Britain's Trident nuclear weapons system, uttered at the end of shopping list of future policies during his Mansion House speech on Wednesday night, has had an explosive effect on the Labour's party's left wing. Why this should be so is not altogether clear. Labour's manifesto at last year's general election included a commitment to retain the nuclear deterrent although it is true that it also said that this should not be finalised before a full debate within the party. That debate is already taking place and the government will have to be careful to ensure that it does not dominate the September party conference to the exclusion of other important issues. Mr Blair will also have to decide whether or not to make the decision dependent on a vote in the House of Commons. It will not only be dyed–in–the–wool opponents of nuclear weapons who oppose a new system to replace Trident at a cost of 25 billion pounds. British governments call Trident an independent deterrent and believe that it adds to Britain's status in the world, but the truth is that this weapon depends on US technical support and would never be used without American agreement. However other regimes with nuclear ambitions, in Iran and North Korea for instance, will notice the importance that the possession of a nuclear weapon is thought to bring even if the likelihood of ever using it is minimal.