Now that President Obama has set a promising arms reduction agenda with President Dmitri Medvedev of Russia, he faces the greater challenge of getting his own government and the American nuclear weapons establishment to support his audacious plan to make deep weapons cuts, Philip Taubman writes in the New York Times.

As President Obama will soon discover, erasing the nuclear weapons legacy of the cold war is like running the Snake River rapids in Wyoming — the first moments in the tranquil upstream waters offer little hint of the vortex ahead. Now that Mr. Obama has set a promising arms reduction agenda with President Dmitri A. Medvedev of Russia, he faces the greater challenge of getting his own government and the American nuclear weapons establishment to support his audacious plan to make deep weapons cuts and ultimately eliminate nuclear weapons.

So far, Mr. Obama has effectively coupled an overarching vision of getting to a world without nuclear weapons, outlined in a speech in Prague earlier this year, with concrete first steps like the one-quarter reduction in operational strategic nuclear weapons promised in Moscow this week. Given his short time in office, and the looming December expiration of the treaty with Russia covering strategic nuclear arms reductions, the new limits are a good, realistic start. It is especially important to extend the monitoring and verification provisions of the expiring arms accord.

But the overall Obama approach involves a balancing act that requires him to move boldly while reassuring opponents that he is not endangering our security. Put simply, he has to maintain a potent nuclear arsenal while slashing it.

Mr. Obama might consider Ronald Reagan’s experience when he tried to set a similar course. The nuclear weapons crowd practically disowned Reagan when he proposed abolishing nuclear weapons during his 1986 summit meeting with Mikhail Gorbachev in Reykjavik, Iceland. After the meeting, when Reagan asked his generals to explore the ramifications of possibly sharply cutting warheads and eliminating nuclear-tipped ballistic missiles, they politely but firmly told their commander in chief it was a terrible idea.

Mr. Obama’s moment of truth with his generals is coming later this year when the Pentagon completes its periodic Nuclear Posture Review. This, in the Pentagon’s words, “will establish U.S. nuclear deterrence policy, strategy and force posture for the next 5 to 10 years.” So it will be the American nuclear weapons bible for the remainder of Mr. Obama’s presidency, one term or two.

President Obama must make sure it reflects his thinking. That will not be automatic, because the nuclear weapons complex — the array of Pentagon and Energy Department agencies involved in nuclear operations, including the armed services and the weapons labs — harbors considerable doubt about his plans. The same goes for the wider world of defense strategists. There is resistance in Congress, too.

The view in these quarters is that the weapons cuts Mr. Obama envisions — deeper than the modest goals set in Moscow this week — would dangerously undermine the power of America’s arsenal to deter attacks against the United States and its allies. Sentiment also favors building a new generation of warheads, a step Mr. Obama has rejected.

If the White House does not assert itself, the Nuclear Posture Review could easily spin off in unhelpful directions. The review that was produced when Bill Clinton was president in 1994 offered a rehash of cold war policies. The one that was done when George W. Bush took office in 2001 was more unconventional, but was quickly overshadowed by the terror attacks of Sept. 11, 2001, and the war in Iraq.

To serve Mr. Obama’s interests, the new review should lay the groundwork for pronounced cuts in weapons and shape America’s nuclear stockpile to fit a world in which threats are more likely to come from states like North Korea and Iran than from a heavily armed power like Russia.

After the review, the next big test for Mr. Obama will likely be Senate consideration of the Comprehensive Test Ban Treaty. He has pledged to resubmit this 1996 United Nations treaty, which was flatly rejected by the Senate in 1999.

To get the two-thirds majority needed for its approval, Mr. Obama will need to hold his fellow Democrats in line — far from a sure thing — and also pick up some Republican support. Two influential Republican senators — John McCain and Richard Lugar — are pivotal. Both voted against the treaty in 1999.

Opponents wrongly argue that the treaty is unverifiable. That might have been the case a decade ago, but technological advances make monitoring of even small underground nuclear tests possible today. Critics also say a permanent ban on testing — the United States has honored a moratorium since 1992 — would eventually cripple the nation’s ability to maintain reliable warheads. So far, most weapons experts would say, that has not proven to be true and should not be for many years.

Few presidential moments are more glittering than the announcement of arms reduction accords in the Kremlin’s gilded halls. For Mr. Obama, that was the easy part.