LONDON (Reuters) - U.S. Secretary of State Hillary Clinton had just left a hotel for a historic signing ceremony between Turkish and Armenian leaders on Saturday when her caravan stopped and she headed back.

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A decades-old conflict, stemming from the World War One mass killing of Armenians by Ottoman forces, needed a few more hours to simmer, it seemed, before resolution could begin.

Clinton, President Barack Obama’s most prominent foreign policy adviser, played a key role in getting the long-term enemies to sign, hours late, protocols to establish diplomatic ties and open borders, U.S. officials said.

She drove back to the hotel where Armenian Foreign Minister Edward Nalbandian was ensconced, spoke to him and Turkish counterpart Ahmet Davutoglu by phone, and persuaded both sides to meet at the university where the signing was to take place, bringing the Armenian along in her sedan.

The holdup? A disagreement over what the leaders would say at the ceremony. The solution? Pledging, in the end, to say nothing -- at least on site.

“There was an agreement that the protocols should speak for themselves,” Clinton told reporters on her plane later in the evening as she made her way to London.

“They’ve been carefully, painstakingly negotiated over many months and at the end of the day that was what the substance of this is about.”

Critics say Clinton is sidelined in Obama’s administration on big issues such as Iran, Afghanistan and Middle East peace, but her European trip to Switzerland, Britain, Ireland and Russia underscored areas in which she has taken the lead.

The former senator’s role in the Turkey-Armenia dispute, which also featured foreign policy heavyweights from Russia and the European Union, illustrated her negotiating muscle several months after she swapped her political job for that of top U.S. diplomat.

“It’s just what you sign up for,” Clinton said when asked whether this was one of the hardest diplomatic challenges she had faced in her short tenure as secretary of state.

“When you’re trying to help people resolve long-standing problems between themselves, it is a very challenging process.”

Clinton declined to identify the sticking points in the talks, though some analysts said Armenia had probably sought to use the word “genocide” when referring to the killings while Turkey may have planned to refer to a dispute between ally Azerbaijan and Armenia over the enclave of Nagorno-Karabakh.

“There were concerns on both sides,” Clinton said. “So there were several times when I said to all of the parties involved that this is too important, this has to be seen through, you’ve come too far.”

Obama, who has also been personally involved in encouraging both sides to sign the protocols, called Clinton as she drove to the airport later to congratulate her, one adviser said.