Cluster munitions explode in midair, releasing dozens, sometimes hundreds of tiny bomblets. They have been documented as causing many civilian deaths during wartime and leaving behind deadly unexploded devices that take lives even after combat has ended.

Cluster munitions explode in midair, releasing dozens, sometimes hundreds of tiny bomblets. They have been documented as causing many civilian deaths during wartime and leaving behind deadly unexploded devices that take lives even after combat has ended. The Pentagon justified their use in a July 9 statement saying cluster munitions are “legitimate weapons with clear military utility” that can “save U.S. lives.”

However, for supporters of the Cluster Munitions Convention, the policy memo was a potentially dangerous response to a landmark agreement. The weapons that the U.S. intends to retain for the next 10 years are “proven killers of civilians,” said Bonnie Docherty, a lecturer on international humanitarian law at Harvard Law School and a participant in the campaign for the cluster munitions convention.

The United States’ own use of the bombs in Iraq in 2003 “caused hundreds of civilian casualties,” she said. And the promises of near perfect detonation rates have proven fallible in the recent past. The “most touted weapon,” said Docherty, “has been the M85, which has been used by Israel in Lebanon. In the field, it had a 10-percent dud rate as opposed to the advertised 1 percent.”

Mark Hiznay, a spokesman for the International Campaign to Ban Landmines, said these failures are evidence of a problem with cluster munitions that may not be soluble.

“In perfect laboratory conditions, it may be possible to achieve a 1 percent failure rate. In the field, under operational conditions, in various climates, the failure rate of submunitions is invariably higher than is claimed by manufacturers or government scientists. Every time cluster munitions are used, we hear that, ‘failure rates were higher than anticipated.’ That is why a comprehensive prohibition is inherently stronger than a purely technical solution,” Mr. Hiznay said.

The memo is the latest instance of a U.S. cluster munitions policy that publicly insists on the importance of the weapons but in practice has begun to shun them. In 2001, the defense secretary at the time, William Cohen, issued a policy memo requiring that all cluster munitions procured by the U.S. after 2005 have a failure rate below 1 percent. This standard has eluded manufacturers, making the U.S. virtually incapable of acquiring new cluster bombs for the past three years.

The U.S. deployed cluster munitions during the invasions of Afghanistan and Iraq, but has not used them in either country since 2003. During the recent negotiations over the convention, the U.S. pressured its allies to word the treaty in such a way as to permit military cooperation between signatories and non-parties in exercises that use cluster munitions.

However, the overall content of the treaty was a defeat for U.S. policy goals, which would have rather preserved the legitimacy of cluster munitions. Instead, more than 100 nations resolved to stigmatize the weapon. The force of the treaty was even more jarring to the U.S. because principal allies such as Britain and Japan agreed not only to ban cluster munitions but to pressure other states to do so as well.