WASHINGTON -- In his first term, President Obama presided over an administration known for its lack of open dissension on critical foreign policy issues.
But on Thursday, deep divisions over what to do about one of those issues -- the rising violence in Syria -- spilled into public view for the first time in a blunt exchange between Senator John McCain, Republican of Arizona, and the leaders of the Pentagon.
Testifying before the Senate Armed Services Committee, Defense Secretary Leon E. Panetta acknowledged that he and the chairman of the Joint Chiefs of Staff, Gen. Martin E. Dempsey, had supported a plan last year to arm carefully vetted Syrian rebels. But it was ultimately vetoed by the White House, Mr. Panetta said, although it was developed by David H. Petraeus, the C.I.A. director at the time, and backed by Hillary Rodham Clinton, then the secretary of state.
"How many more have to die before you recommend military action?" Mr. McCain asked Mr. Panetta on Thursday, noting that an estimated 60,000 Syrians had been killed in the fighting.
And did the Pentagon, Mr. McCain continued, support the recommendation by Mrs. Clinton and Mr. Petraeus "that we provide weapons to the resistance in Syria? Did you support that?"
"We did," Mr. Panetta said.
"You did support that," Mr. McCain said.
"We did," General Dempsey added.
Neither Mr. Panetta nor General Dempsey explained why President Obama did not heed their recommendation. But senior American officials have said that the White House was worried about the risks of becoming more deeply involved in the Syria crisis, including the possibility that weapons could fall into the wrong hands. And with Mr. Obama in the middle of a re-election campaign, the White House rebuffed the plan, a decision that Mr. Panetta says he now accepts.
With the exception of General Dempsey, the officials who favored arming the rebels have either left the administration or, as in Mr. Panetta's case, are about to depart. Given that turnover, it is perhaps not surprising that the details of the debate -- an illustration of the degree that foreign policy decisions have been centralized in the White House -- are surfacing only now. A White House spokesman declined to comment on Thursday.
The plan that Mr. Petraeus developed, and that Mrs. Clinton supported, called for vetting rebels and training a cadre of fighters who would be supplied with weapons. The plan would have enlisted the help of a neighboring state.
The proposal offered the potential reward of creating Syrian allies for the United States during the conflict and if President Bashar al-Assad is removed.
Some administration officials expected the issue to be revisited after the election. But when Mr. Petraeus resigned because of an extramarital affair and Mrs. Clinton suffered a concussion, missing weeks of work, the issue was shelved.
Syria rebel leaders have long appealed for weapons. Mohammad Hussein al-Haj Ali, a Syrian major general who defected to the opposition, said in a telephone interview last year that he had raised the issue of arming the resistance in a September meeting in Amman, Jordan, with Gen. James N. Mattis, the head of the Central Command, which oversees operations in the Middle East.
"He was very sympathetic to it, but his main concern was who would actually get hold of these weapons," he said, referring to General Mattis.
General Haj Ali said he promised that the rebels who were armed would take care not to lose control of the weapons and would return any that they did not use.
General Mattis "said he would meet the top administration officials within 48 hours and get back to me," General Haj Ali said through an interpreter, adding that he still had received no response several weeks later.
The debate over arming the rebels is complex and turns on assessments on the military advantages they might gain, the political calculations on who might come to power in Syria, and the dangers that the arms might fall into the wrong hands.
Jeffrey White, a former senior analyst with the Defense Intelligence Agency and a fellow at the Washington Institute for Near East Policy, said that providing weapons would help the rebels in their fight against a better-equipped government that has warplanes, armor and artillery, and reduce rebel casualties.
Equally important, Mr. White said, it would give the United States influence with groups that would control Syria if Mr. Assad is ousted, and would diminish the influence of extremists.
"The day after the regime falls, the groups that have the guns will dominate the political and military situation," Mr. White said. "And if some of those groups owe that capability to us, that would be a good thing. It does not mean that we would control the situation, but it would give us a means of shaping it."
Taking a contrary view, Daniel C. Kurtzer, a former United States ambassador to Israel and Egypt, said the potential risks outweighed the gains. Even with thorough vetting, he said, it would be difficult to ensure that the weapons did not end up with unreliable or hostile groups.
"The problem that I think the White House has identified much more clearly than the national security team is, 'Who are you going to deal with?' " Mr. Kurtzer said.
Much of the lengthy hearing was devoted to sparring over the Pentagon's response to the Sept. 11 attack on the United States diplomatic compound in Benghazi, Libya. The Pentagon chiefs said the military was not able to respond faster because there was no intelligence of an imminent attack. Mr. McCain faulted the Pentagon for not positioning forces in the region before the anniversary of the Sept. 11, 2001, attacks.
But the statements by the Pentagon chiefs on Syria were so striking that Senator Lindsey Graham, Republican of South Carolina, returned to them after a break in the proceedings.
"Both of you agreed with Petraeus and Clinton that we should start looking at military assistance in Syria, is that correct?" Senator Graham asked.
"That was our position." Mr. Panetta said. "I do want to say, Senator, that obviously there were a number of factors that were involved here that ultimately led to the president's decision to make it nonlethal.
"And I supported his decision in the end," Mr. Panetta continued. "But my answer to your question is yes."
Mr. McCain said he was dismayed that Mr. Obama had "overruled the senior leaders of his own national security team, who were in unanimous agreement that America needs to take greater action to change the military balance of power in Syria."
This article originally appeared in The New York Times.