Gordon Brown said yesterday that Britain would sign a treaty banning cluster munitions. The treaty is being formulated at an international conference in Dublin. It has weighty supporters. “Such a treaty will establish a new benchmark for the responsible projection of force in the modern world,” wrote nine leading military figures in a letter to this newspaper.

They are wrong. The case for a “new benchmark” is a fallacy that will undermine collective security. The treaty's greatest impact will be not in protecting civilians but in hampering the military capability of the states that are most scrupulous in limiting the destructiveness of warfare.

Cluster munitions are weapons that subdivide into smaller bombs when fired. They then disperse across a wide area. Sometimes these bomblets fail to explode. If they are disturbed later, they may maim or kill civilians.

There is one state, Laos, where unexploded ordnance remains a serious threat to civilians. But Laos is an exception. Last year confirmed casualties throughout the world from cluster munitions amounted to fewer than 400. These are individual tragedies. They do not amount, in aggregate, to anything approaching the humanitarian issue raised by, say, the trade in small arms.

The diplomatic efforts to ban cluster munitions, moreover, have costs. These weapons are not indiscriminate, and they do have a place in warfare. They are effective against moving or dispersed targets such as tank formations and airfields. If they are not used against such targets, then something else will be: probably rocket barrages or massed artillery. These have a humanitarian impact too - in civilian lives and in destroying infrastructure - and there will be more of them.

Campaigners might ask themselves whether the best means of limiting the civilian casualties of cluster bombs is in increasing the weapons' reliability and precision rather than banning them.

The most enduring costs of an extensive treaty, however, will be to the solidarity of Nato. The United States insists on the option of using cluster munitions. The US is not merely one state among many. In the absence of world government, it is the provider and guarantor of collective security. Under the terms of the treaty, military personnel might face criminal prosecution if they operated alongside US forces.

Collective security is the foundation of our defence policy. This ostensibly humanitarian campaign strikes at the heart of it, to the future benefit of aggressors everywhere.

Oliver Kamm is the author of Anti-Totalitarianism: the Left-Wing Case for a Neoconservative Foreign Policy

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