Every four years the White House issues a “nuclear posture review.” That may sound like an anachronism. It isn’t. In a world where the United States and Russia still have more than 20,000 nuclear weapons — and Iran, North Korea and others have seemingly unquenchable nuclear appetites — what the United States says about its arsenal matters enormously.

President Obama’s review was due to Congress in December. That has been delayed, in part because of administration infighting. The president needs to get this right. It is his chance to finally jettison cold war doctrine and bolster America’s credibility as it presses to rein in Iran, North Korea and other proliferators.

Mr. Obama has already committed rhetorically to the vision of a world without nuclear weapons. But we are concerned that some of his advisers, especially at the Pentagon, are resisting his bold ambitions. He needs to stick with the ideas he articulated in his campaign and in speeches last year in Prague and at the United Nations.

These are some of the important questions the posture review must address:

THEIR PURPOSE: Current doctrine gives nuclear weapons a “critical role” in defending the United States and its allies. And it suggests they could be used against foes wielding chemical, biological or even conventional forces — not just nuclear arms. Mr. Obama’s aides have proposed changing that to say that the “primary” purpose of nuclear weapons is to deter a nuclear attack against the United States or its allies. This still invites questions about whether Washington values — and might use — nuclear forces against non-nuclear targets.

Given America’s vast conventional military superiority, broader uses are neither realistic nor necessary. Any ambiguity undercuts Washington’s credibility when it argues that other countries have no strategic reason to develop their own nuclear arms. The sole purpose of American nuclear forces should be to deter a nuclear attack against this country or its allies.

HOW MANY: President George W. Bush disdained arms control as old think, and Washington and Moscow have not signed an arms reduction treaty since 2002. Mr. Obama launched negotiations on a new agreement that would slash the number of warheads each side has deployed from 2,200 to between 1,500 and 1,675. The talks are dragging on, but there is hope for an agreement soon. Both sides should go deeper.

The review should make clear that the United States is ready to move, as a next step, down to 1,000 deployed warheads — military experts say half that number is enough to wipe out the assets of Russia, which is no longer an enemy. China, the only major nuclear power adding to its arsenal, is estimated to have 100 to 200 warheads. The treaty being negotiated says nothing about the nearly 15,000 warheads, in total, that the United States and Russia keep as backups — the so-called hedge. And it says nothing about America’s 500 short-range nuclear weapons, which are considered secure, or Russia’s 3,000 or more, which are chillingly vulnerable to theft.

The review should make clear that there is no need for a huge hedge, and that tactical weapons have an utter lack of strategic value — as a prelude to reducing both. Certainly no general we know of could imagine exploding a warhead on a battlefield. Today’s greatest nuclear danger is that terrorists will steal or build a weapon. That is best countered by halting proliferation and securing and reducing stockpiles and other material.

NEW WEAPONS: The United States built its last new warhead in 1989. So when aides to President George W. Bush called for building new weapons, with new designs and new capabilities, it opened this country to charges of hypocrisy and double standards when it demanded that North Korea and Iran end their nuclear programs.

Mr. Obama has said that this country does not need new weapons. But we are concerned the review will open the door to just that by directing the labs to study options — including a new weapons design — for maintaining the arsenal. The government has a strong and hugely expensive system for ensuring that the stockpile is safe and reliable. Mr. Obama has already vastly increased the labs’ budgets. The review should make clear that there is no need for a new weapon.

ALERT LEVELS: The United States and Russia each still have about 1,000 weapons ready to fire at a moment’s notice. Mr. Obama has rightly described this as a dangerous cold war relic. The review should commit to taking as many of those forces off hair-trigger alert as possible — and encourage Russia to do the same.

In April, Mr. Obama will host a much needed summit meeting on the need to better secure nuclear material from terrorists. In May, Washington will encourage a United Nations-led conference to strengthen the Nuclear Nonproliferation Treaty, the bedrock, and battered, agreement for curbing the spread of nuclear arms.

President Obama will also have to persuade the Senate to ratify the Start follow-on treaty, and we hope he will quickly press the Senate to approve the test ban treaty. He is also working with allies to revive nuclear talks with North Korea and to impose tougher sanctions on Iran. Getting the nuclear posture review right is essential for moving all of this ahead.