No one has more invested in cluster munitions than the United States, which Human Rights Watch says has been the largest producer, stockpiler and user, using them in the former Yugoslavia, Afghanistan and Iraq. Others that have used them include Britain, France, Sudan, NATO, Israel and Hezbollah.

United States officials insist the Pentagon must have such munitions. That is what the Clinton administration said when it opposed the land-mine treaty in 1997. It is a weak argument: cluster bombs are weapons for conventional wars with conventional battlefields. America is less likely to fight big conventional wars than counterinsurgency conflicts in population centers, no place for munitions that kill indiscriminately.

As the main holdout, the United States gives cover to countries like Russia and China, which also rejected the ban. The treaty is weaker for it: together, these three nations have more than a billion cluster munitions stockpiled, far more than the number of weapons expected to be destroyed. Also weakening the pact is a loophole that will let America continue military cooperation with treaty signers, even if it uses cluster munitions.

At least this treaty, like the land-mine ban, will stigmatize cluster munitions and make it harder to use them. Since the land-mine treaty entered into force, experts say more than 40 million have been destroyed, trade in land mines has virtually ended, and in 2007 only two countries  Russia and Myanmar  used them. The United States has paid $1.2 billion (more than any other nation) to defuse land mines and clean up war zones.

Modern nations need a range of weapons to protect their legitimate interests. Cluster munitions that disproportionately harm civilians are not among them. President Bush must resist the temptation to further sabotage this worthy treaty and let it take effect. It is not clear where the candidates stand on the treaty, but the next president, whoever it is, should repudiate Mr. Bush’s opposition and sign it.