Lakhdar Brahimi, who gave up Tuesday after trying unsuccessfully to mediate an end to Syria's civil war, has spent his career in the diplomatic trenches of the world's most intractable conflicts.
The 80-year-old Algerian succeeded in cajoling President Bashar al-Assad's regime and its fractious opponents to two rounds of peace talks in Geneva this year but failed to halt the civil war, which has killed more than 150,000 people and displaced millions since March 2011.
UN chief Ban Ki-moon announced the resignation in New York,with Brahimi standing beside him, saying it would be effective from May 31.
Brahimi, one of the world's most respected veteran envoys, had been involved in high-stakes diplomacy for long enough not to set expectations very high.
"I'm not disappointed, because I did not expect any result this first time," he said of the glacial pace of talks back in January, warning that there was no "magic wand."
But in the months since the gulf between the two sides has grown even wider, with Syria scheduling a presidential election for June 3 despite Brahimi's warning that doing so would sound the death knell for the negotiations.
And the United States and Russia, which had strongly backed Brahimi's efforts to bring the two sides to the negotiating table, are now bitterly split over the crisis in Ukraine.
Brahimi was known for bringing patience, pragmatism and a healthy dose of humour to the many impossible missions he embarked on, and Syria was no different.
"At this rate, we'll need 20 years. You'd better hurry up, as I won't be around in 20 years," he reportedly told the Syrian delegates in Geneva.
As a veteran of Algeria's war of independence against colonial ruler France over five decades ago, and later as foreign minister, Brahimi carries weight in the Middle East, and was the envoy of both the United Nations and the Arab League.
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"The fact that he speaks Arabic, the fact that he understands the culture, must play a role," Syrian opposition spokeswoman Rafif Jouejati told AFP.
- 'Limitless patience' -
Brahimi, also fluent in French and English, earned his spurs in 1989 by helping the Arab League broker the deal that ended Lebanon's 15-year civil war.
He was UN envoy to South Africa during the watershed 1994 elections that saw Nelson Mandela win office, then moved on to civil war-torn Yemen.
After serving as UN envoy to Afghanistan following the September 11, 2001 attacks on the United States, he was sent to Iraq after the 2003 US-led invasion which ousted Saddam Hussein.
Former war correspondent Harriet Martin, who profiled Brahimi in her 2006 book on mediators, "Kings of Peace, Pawns of War", described him as a charming, clever strategist with a dry sense of humour.
"He also has a quiet sense of his own power –- and the limits of it. Unlike other mediators, he is not intimidated by the big diplomatic power games he is often caught in the middle of," she told AFP.
"For a man of 80, his patience is limitless," said Ghassan Salame, dean of the School of International Affairs in Paris, who knows Brahimi well.
"He looks for the soft spots of belligerents and works towards the final goal despite the rocky road".
In the case of Syria, however, that road seemed to hit a dead end, with the regime following up on a string of battlefield victories with a controversial election, the opposition plagued by infighting and the international community as divided as ever.