When Mr. Petraeus was the commander of forces in Iraq and then-Senator Clinton was serving on the Senate Armed Services Committee and preparing for her 2008 presidential bid, she had all but called him a liar for trumpeting the military gains of the troop increase ordered by President Bush. But serving together in the Obama administration, they were allies when it came to Syria, as well as on the debate over how many troops to send to Afghanistan at the beginning of the administration.

Mr. Petraeus had a background in training foreign forces from his years in Iraq, and his C.I.A. job put him in charge of covert operations. The Americans already had experience in providing nonlethal assistance to some of the rebels.

The plan that Mr. Petraeus developed and Mrs. Clinton supported called for vetting rebels and establishing and arming a group of fighters with the assistance of some neighboring states. Defense Secretary Leon E. Panetta was said by some officials to be sympathetic to the idea. Mr. Petraeus and a spokesman for Mr. Panetta declined to comment.

Wary of becoming entangled in the Syria crisis, the White House pushed back, and Mrs. Clinton backed off. Some administration officials expected the issue to be joined again 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.

In an interview last week, Mrs. Clinton declined to comment on her role in the arms debate and emphasized other steps the United States had taken. “We have worked assiduously, first to create some kind of legitimate opposition,” she said. “We have been the architect and main mover of very tough sanctions against Assad.”

She added: “Having said all that, Assad is still killing. The opposition is increasingly being represented by Al Qaeda extremist elements.” She also said that the opposition was getting messages from the ungoverned areas in Pakistan where some of the Qaeda leadership was believed to be hiding — a development she called “deeply distressing.”

If Syria and Benghazi were low points for Mrs. Clinton, then the diplomatic opening to the military government in Myanmar, also known as Burma, was perhaps the biggest highlight. There, too, she initially met resistance from the White House and Pentagon, as well as the prospect of opposition from the Senate minority leader, Mitch McConnell of Kentucky, a stalwart supporter of Myanmar’s prodemocracy leader, Daw Aung San Suu Kyi.