But scholars and diplomatic insiders say she has never dominated issues of war and peace in the manner of predecessors Dean Acheson or Henry Kissinger, or laid down an enduring diplomatic doctrine.

President Barack Obama has tightly controlled US foreign policy in the past four years - more so than his recent predecessors.

Clinton has had a seat at the table on every key issue, officials say, but she did not "own" any of them.

She worked hard on signature issues such as the empowerment of women, gay rights, third world development, health and internet freedoms.

Clinton lent her support to a wide range of new projects and organisations, and she appointed new officials in the State Department to shepherd them.

Some of these may eventually have huge effects, but many are at an early stage.

John Cassidy, of , wrote: "Perhaps her most memorable moment was helping to secure the freedom of Chen Guangcheng , the Chinese dissident, who is now a scholar in residence at New York University.

"As a 'rock star diplomat', she toured tirelessly and put on good shows. Since that's what she was hired to do, it seems a bit unfair to judge her too harshly."

At the same time, the most important and toughest foreign policy issues of the day - Iran, North Korea, Afghanistan and Pakistan, the Arab-Israeli stand-off - weren't resolved during the four years.

Some grew more intractable. While none of that may be Clinton's fault, the lack of diplomatic breakthrough on her watch limits her legacy.

Writing in , Michael O'Hanlon, from the Washington-based research group The Brookings Institution, said she has pushed a successful "pivot" towards Asia and built on strong European relationships.

Yet her often cautious approach has also meant that she was rarely imaginative in the role and "few big problems were solved on her watch".

Expectations ran high that Clinton would be a heavyweight - maybe even a "co-president" on foreign policy - from the time Obama picked his bitter rival in the 2008 Democratic presidential primary campaign to take the cabinet's senior spot.

She had star power from 20 years in national life that dazzled foreign audiences and guaranteed worldwide attention to whatever issue she focused on.

"She's the first secretary who's also been a global rock star," said a senior State Department official.

"It's allowed her to raise issues on the global agenda in a way that no one before her has been able to do."

As secretary of State, Clinton has shared Obama's democratic take on the proper role of American diplomats, believing that the world is no longer a place where a handful of powers can dictate the terms of the world order.

Rather, the job of US diplomats is collaborating with dozens of other countries in the "constant gardening and tending" of institutions and projects that advance common goals, the senior State Department official said.

Foreign audiences warmed to this attitude, which they found appealing after eight years of a George W. Bush administration many associated with a go-it-alone approach. As they did, the American image abroad improved.

At the same time, Clinton quickly removed a potential internal stumbling block, insisting on no infighting between her loyalists at the State Department and Obama's team.

Former president Bill Clinton's interference on foreign policy never became the problem some had predicted.

A hard worker and team player, Clinton won praise from many in Obama's circle who had initially doubted her. But as time passed, it became clear that she wouldn't have the lead role on key issues of war and peace.

Clinton's original plan was to have three powerful "special envoys" in charge of key security issues and reporting to her - a flow chart that would have enabled her to tightly control the biggest security issues.

But Richard Holbrooke, in charge of the Afghanistan-Pakistan militant threat, was marginalised after clashing with White House officials. Former Senate Democratic leader George Mitchell resigned in May 2011 after the painful collapse of the administration's opening Middle East peace initiative.

And diplomat Dennis Ross, the envoy for Iran, moved to the White House in June 2009 to better help manage the range of Middle East problems that were bubbling over.

"She was a fully functioning member of the team," said a former administration official, who asked to remain anonymous. "But not a first among equals.

"If you go down the line, it's tough to see what's happened in world politics over the last four years that wouldn't have happened without her.

"So it's tough to see how she gets into that category of truly great, transformational secretaries, like Acheson and [George] Marshall," who presided over US foreign policy in the years after the second world war.

Through most of the four years, she seemed immune to criticism. The most notable exception came near the end, with the death of four Americans at the US mission in Benghazi, Libya, in September.

But although Clinton said she took responsibility for security weaknesses there, Republican criticism largely focused not on her, but on Susan Rice, the US ambassador to the United Nations, who had appeared on a Sunday morning talk show a few days after the killings to defend the administration's handling of the matter.

Supporters of Hillary Clinton for president have already created a super-political action committee in case she decides to run for the White House in 2016.

Ready for Hillary, one of two Clinton super-PACs registered with the Federal Election Commission, says on its Facebook page that it was created by five friends, including a former aide and former adviser to the New York Democrat.

Asked in an interview about her future, Clinton said: "I am still secretary of state. So I'm out of politics."

But she added later: "I can't make predictions about what's going to happen tomorrow or the next year."