GENEVA - Believe the advocates of a treaty banning cluster munitions, and the international community is about to take a decisive step toward curbing the use of a weapon that inflicts terrible suffering, particularly on civilians. Believe the US government, and the measure they propose threatens to undermine the NATO alliance that has underpinned Western security since World War II.

Delegates from more than 100 countries will open a conference in Dublin tomorrow that will try to hammer out a treaty banning the production, use, stockpiling, or transfer of cluster munitions - bombs or artillery shells packed with up to several hundred bomblets or submunitions that are sprayed over wide areas of territory.

Major producers and stockpilers of cluster munitions, the United States, Russia, and China, will be absent and are opposed to a treaty, but disarmament specialists liken the cluster treaty to the Ottawa Treaty of 1997 banning land mines, which was shunned by the major powers but has proved influential in shaping the policies of countries outside the convention.

Support for a ban on cluster weapons has risen sharply since the 2006 conflict between Israel and Lebanon, when, according to United Nations estimates, Israeli troops fired some 4 million Vietnam War-era submunitions, of which a quarter failed to explode.

These have reportedly caused more than 200 casualties since the end of the war and required a costly and hazardous cleanup operation by international aid agencies - often funded by Western governments.

In Laos, where the United States dropped 2 million tons of ordnance in the 1970s, including an estimated 260 million submunitions, unexploded weapons still kill and maim people and hinder economic development.

As many as 10 percent to 15 percent of cluster munitions normally fail to explode on impact but those who support the treaty say the figure could be much higher. A study by Handicap International, a nongovernmental organization based in Belgium, found that 98 percent of recorded victims were civilians and one-third of casualties were children.

But after a series of international and regional conferences that have mapped out the broad parameters of a treaty, Dublin is the venue where negotiators have to refine rhetoric into a legally binding instrument governing a weapon system that represents a substantial part of the arsenal of the United States and some of its NATO allies.

"It's going to be a bruising conference," said Patrick McCarthy, coordinator of the Geneva Forum, a disarmament research body.

A handful of issues loom as key battlegrounds. One will be the definition of what constitutes a cluster munition, with richer Western nations like Britain, Denmark, France, Germany, Italy, Sweden, and Switzerland pressing for exclusion of sophisticated weapons that have self-destruct mechanisms, target sensors, or a small number of submunitions.

Others, like Germany, want a transition period of up to 10 years during which they can continue to use such weapons while they find replacements.

Among the most contentious is a proposed clause that would prevent those who sign onto the treaty from engaging in joint operations with forces still employing cluster munitions.

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