Britain has decided to renew its nuclear deterrent. A parliamentary vote last month sharply divided the ruling Labor Party, forcing Prime Minister Tony Blair to rely on opposition Conservatives to pass the measure. Mr. Blair made his case by arguing that international uncertainty required Britain to be prepared for all security threats.

And, in the odd logic of nuclear strategy, the prime minister also asserted that building the next generation of nuclear weapons-carrying submarines better enables Britain to push for multilateral disarmament.

Britain has been a nuclear-weapons state since the mid-1950s. After deciding that its nuclear bombers were vulnerable to Soviet air defenses, London moved to a submarine-launch deterrent in 1968. Twelve years later, the British government built its second generation of submarines, the Vanguard class, the first of which was launched in 1992.

Britain now has four Vanguard-class submarines, each of which carries 16 U.S.-made Trident missiles. Each missile has between three to eight warheads. Each submarine has a life expectancy of 25 years.

Mr. Blair argues that Britain must decide now whether to proceed with the next-generation submarine, which could take up to 17 years to design, build and deploy. A decision putting off this move would risk losing the skilled labor needed to build those submarines. The credibility of Britain’s deterrent rests on action taken today.

Nuclear weapons have long been a source of profound disagreement within the Labor Party. For many years, the party platform demanded unilateral disarmament by the British government. The commitment of many party members to the cause has not flagged. The result has been one of the biggest defections of Labor backbenchers that Mr. Blair has seen since he took office.

In the vote, nearly 100 Labor parliamentarians — about one-quarter of the party bloc — voted against replacing the submarines. Four high-profile members, including the deputy Commons leader, resigned their jobs in protest. The measure passed nonetheless with the support of opposition Conservative Party members who harbor no doubts about the utility of the British nuclear force. To appease disgruntled parliamentarians, Mr. Blair suggested that he might cut the number of submarines from four to three and promised to reduce the number of nuclear warheads in its arsenal by 20 percent to fewer than 160. If that promise is kept, Britain’s arsenal — the smallest among the five declared nuclear-weapons states — will have shrunk by 75 percent since the end of the Cold War.

Dissenters object to the weapons on two grounds. One is cost. They challenge the government estimate that the submarines will cost only 15 billion euro to 20 billion euro. They say the total is more likely to reach 100 billion euro and argue that the money would be better spent on domestic needs such as health care.

The more powerful objection is that nuclear weapons are immoral. Any use would be catastrophic and the damage they create would be disproportionate to any possible good.

Worse, Britain’s decision to renew its arsenal sends the wrong message at a time when the world is trying to convince countries like North Korea and Iran to abandon their nuclear ambitions. If Britain might need such weapons — as Mr. Blair asserted — then other countries might, as well. To suggest that nuclear weapons have military utility can only inspire other governments to want them too.

Mr. Blair counters that a decision not to build new weapons would constitute unilateral disarmament and would undo whatever leverage Britain has in multilateral disarmament talks: London would give up the weapons it could trade for cuts from other nuclear-weapons states.

Mr. Blair may have other political calculations in mind too. He has vowed to step down later this year, and pushing through a vote on the submarines now spares his successor a divisive encounter early in his or her tenure. That intention may be undermined by lingering ill will in his party. Even some of Mr. Blair’s supporters argue that the debate on the new submarines was rushed through Parliament. The next prime minister will still have to rely on legislators who feel slighted by a lack of proper debate on a critical national concern.

The Labor Party has grown increasingly fractious in recent years, and rebels have become increasingly emboldened in recent months. This vote will only increase their animus. Nuclear weapons continue to confound both friends and foes.

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