A mood of weary fatalism tends to hover around many of the international diplomats charged with trying to nudge the Democratic Republic of Congo towards a lasting peace in the war-ravaged east of the country.
But today, Russell Feingold allowed himself a smile and a bracing dose of optimism as an aide whispered in his ear to tell him that the rebel group, M23, had just announced a final and immediate end to its two-year insurgency - the latest manifestation of a regional crisis that has endured, with few intervals, for a generation.
"This is a critical and exciting step in the right direction," said Mr Feingold, the United States' Special Envoy for the Great Lakes and DR Congo, who was in Pretoria, South Africa, for a big regional summit on the topic.
He described the enduring instability in DR Congo as "one of the toughest problems in the world", but said "it has never seen such sustained [international] attention".
The M23 was forced to end its rebellion by a combination of factors: Concerted international pressure; an unusually competent performance by DR Congo's armed forces; the robust action of a new UN "intervention brigade" brought in to give the world's largest peacekeeping force sharper military clout and Rwanda's apparent decision to stop its (alleged) military support for the rebels.
"This is a test case - it has great promise," said Mr Feingold of the role played by the new UN force that worked in close co-ordination with DR Congo's military.
"It has enormous potential to add great credibility to UN peacekeeping operations [in other conflict zones]. It has great promise and significance," he said.
Under the terms of an agreement negotiated in neighbouring Uganda, the Congolese authorities must now cease their military activities against M23, while the rebel leadership must - and this was a crucial breakthrough at the peace talks - be held accountable by DR Congo's courts for any serious crimes committed.
Unlike previous deals, there will be no blanket amnesty and no expectation that the rebels can remain in their (often mutinous) units to be reintegrated into the Congolese army.
There is, inevitably, plenty that could go badly wrong in the days ahead. Rebel commanders may try to evade justice. Fighting could flare up again.
Mr Feingold said the US would continue to put pressure on Rwanda to live up to its agreement not to support the M23. The international community "will be watching every minute to make sure each piece of this deal is followed through", he stressed.
And then there are all the other rebel groups and militias, still active in the dense forests of eastern DR Congo, to be dealt with.
"These operations are not simple," said Mr Feingold. The M23 was a "conventional" armed group. Dozens of other "local or guerrilla" forces will "melt back into villages".
"But if we can deal with larger groups then others may see the writing on the wall," he said. "It will be easier for the Congolese government to assert authority and for people to see the benefit of peace. That could have a good domino effect."
The next target for the Congolese army and the UN should, said Mr Feinstein, be the FDLR, the remnants of the ethnic Hutu forces that carried out the 1994 genocide in Rwanda before being forced across the border into DR Congo. Rwanda, he said, had "a right" to expect the group to be disarmed.
In the longer term, stability in DR Congo will also depend on how the Congolese authorities react to what is, for them, an all too rare military victory.
Mr Feingold said recent events were a sign that DR Congo's "President Kabila and the military understand the great need for reform…. to put security sector reforms in high gear".
That includes a change of commanders, military training, and better systems to ensure soldiers get paid.