NEW YORK—When American diplomats sat down for the first in a series of face-to-face talks with their Iranian counterparts last October in Geneva, few would have predicted that what began as a negotiation over Tehran’s nuclear programs would wind up in a stunning demand by the Security Council that Israel give up its atomic weapons.

Yet that’s just what the U.N. body did this morning, in a resolution that was as striking for the way member states voted as it was for its substance. All 10 nonpermanent members voted for the resolution, along with permanent members Russia, China and the United Kingdom. France and the United States abstained. By U.N. rules, that means the resolution passes.

View Full Image

Getty ImagesMahmoud Ahmadinejad meets IAEA Director General Mohamed ElBaradei.

The U.S. abstention is sending shock waves through the international community, which has long been accustomed to the U.S. acting as Israel’s de facto protector on the Council. It also appears to reverse a decades-old understanding between Washington and Tel Aviv that the U.S. would acquiesce in Israel’s nuclear arsenal as long as that arsenal remained undeclared. The Jewish state is believed to possess as many as 200 weapons.

Tehran reacted positively to the U.S. abstention. “For a long time we have said about Mr. Obama that we see change but no improvement,” said Iranian Foreign Minister Manouchehr Mottaki. “Now we can say there has been an improvement.”

The resolution calls for a nuclear weapons-free zone in the Middle East. It also demands that Israel sign the 1970 Nuclear Nonproliferation Treaty and submit its nuclear facilities to international inspection. Two similar, albeit nonbinding, resolutions were approved last September by the International Atomic Energy Agency in Vienna.

At the time, the U.S. opposed a resolution focused on Israel but abstained from a more general motion calling for regional disarmament. “We are very pleased with the agreed approach reflected here today,” said then-U.S. Ambassador to the IAEA Glyn Davies.

Since then, however, relations between the Obama administration and the government of Israeli Prime Minister Benjamin Netanyahu, never warm to begin with, have cooled dramatically. The administration accused Tel Aviv of using “disproportionate force” following a Nov. 13 Israeli aerial attack on an apparent munitions depot in Gaza City, in which more than a dozen young children were killed.

Mr. Netanyahu also provoked the administration’s ire after he was inadvertently caught on an open microphone calling Mr. Obama “worse than Chamberlain.” The comment followed the president’s historic Dec. 21 summit meeting with Iranian President Mahmoud Ahmadinejad in Geneva, the first time leaders of the two countries have met since the Carter administration.

But the factors that chiefly seemed to drive the administration’s decision to abstain from this morning’s vote were more strategic than personal. Western negotiators have been pressing Iran to make good on its previous agreement in principle to ship its nuclear fuel to third countries so it could be rendered usable in Iran’s civilian nuclear facilities. The Iranians, in turn, have been adamant that they would not do so unless progress were made on international disarmament.

“The Iranians have a point,” said one senior administration official. “The U.S. can’t forever be the enforcer of a double standard where Israel gets a nuclear free ride but Iran has to abide by every letter in the NPT. President Obama has put the issue of nuclear disarmament at the center of his foreign policy agenda. His credibility is at stake and so is U.S. credibility in the Muslim world. How can we tell Tehran that they’re better off without nukes if we won’t make the same point to our Israeli friends?”

Also factoring into the administration’s thinking are reports that the Israelis are in the final stages of planning an attack on Iran’s nuclear installations. Defense Secretary Robert Gates, who met with his Israeli counterpart Ehud Barak in Paris last week, has been outspoken in his opposition to such a strike. The Jerusalem Post has reported that Mr. Gates warned Mr. Barak that the U.S. would “actively stand in the way” of any Israeli strike.

“The Israelis need to look at this U.N. vote as a shot across their bow,” said a senior Pentagon official. “If they want to start a shooting war with Iran, we won’t have their backs on the Security Council.”

An Israeli diplomat observed bitterly that Jan. 20 was the 68th anniversary of the Wannsee conference, which historians believe is where Nazi Germany planned the extermination of European Jewry. An administration spokesman said the timing of the vote was “purely coincidental.”

Write to bstephens@wsj.com