U.S. Secretary of State John Kerry holds an award as he speaks at Chatham House in London on Oct. 31. (Peter Nicholls/Reuters)

U.S. Secretary of State John F. Kerry delivered an impassioned defense of diplomacy Monday, asserting that the nuclear deal he spearheaded with Iran had averted an almost-certain war and vowing to work “to the last moment” to achieve a lasting cease-fire in Syria.

In London to accept the Chatham House Prize, given by the prestigious think tank, Kerry said he would “rather be caught trying” to reach seemingly impossible diplomatic agreements than submit to the inevitability of war.

“Believe it or not, there’s nothing inevitable about conflicts raging in places like Syria, Yemen, Libya and South Sudan,” he said. “If war is a choice, then peace is also a choice.”

But after years of futile efforts to stop the conflict in Syria, the secretary also acknowledged just how difficult it will be for the region’s warring parties to end the violence in that ravaged nation.

“Syria is a whole bunch of wars taking place in the same place,” he said, before listing a half-dozen ethnic, sectarian and geopolitical fault lines. “The mix is just about as difficult as any diplomatic cocktail I can think of.”

The president-elect, whether it's Donald Trump or Hillary Clinton, will face the daunting task of dealing with these three global issues. (Jason Aldag,Ishaan Tharoor/The Washington Post)

Kerry was across the Atlantic for less than 36 hours. But he returns to Washington with a trio of awards recognizing his contributions to global peacemaking, including one given in Ireland on Sunday and two in London on Monday.

With just 2½ months left before a new president takes office, Kerry’s visit may be among his last during a half-century-long career in public service that began with a combat tour in Vietnam and included 28 years as a U.S. senator.

The Chatham House Prize was awarded in particular recognition of the Iran deal — perhaps the most high-profile agreement of Kerry’s four-year tenure at State — and was given jointly to Kerry and his Iranian counterpart, Mohammad Javad Zarif.

The deal, reached last year after marathon negotiations in conference rooms across Europe, limits Iran’s nuclear capacity and imposes strict international monitoring in exchange for the lifting of sanctions. It was fiercely opposed by the Israeli government, Republican critics in Congress, Persian Gulf states and hard-liners in Iran.

But it was praised Monday by Philip Hammond, Britain’s former foreign secretary and current treasury chief, as “one of the great achievements of modern diplomacy.” Hammond, who participated with Kerry in the talks, said the secretary had fought “three separate battles” to reach the deal — winning over skeptical parties in Washington, among U.S. allies and in Iran.

Kerry, wearing a red poppy in his suit lapel in a traditional British symbol of remembrance for war veterans, said reaching the deal with Iran had required “an orchestral” diplomatic effort that included members of the P5 + 1, a reference to the five members of the U.N. Security Council plus Germany. He praised Zarif, who did not attend but is expected to pick up his share of the prize at a later date, as “a patriot who fought hard for his nation’s interests.”

He also credited Iran with upholding its end of the bargain, saying the country had “complied with every part” of the deal. He said the United States and its allies had tried to do likewise, but he acknowledged disappointment that European banks have been reluctant to reengage in Iran for fear of running afoul of U.S. law.

Chatham House’s decision to give Kerry and Zarif the prize, which was made through a vote of the organization’s membership, was criticized by opponents of the Iran deal. Rep. Edward R. Royce (R), chairman of the House Foreign Affairs Committee, said the award “only serves to legitimize the radical regime.”

Mark Dubowitz, executive director of the Foundation for Defense of Democracies, said it would have been better to wait until the restrictions on Iran’s nuclear program expire in 10 to 15 years before lauding the deal’s negotiators.

“History is filled with awards given prematurely for accomplishments that end up blowing up in the face of both the recipients and the sponsoring organization,” he said.

Kerry, who was given a scroll signed by Chatham House’s patron, Queen Elizabeth II, said the alternative to a deal would have been war.

Kerry said the Syrian war is a more tangled diplomatic knot than any he has faced, but insisted he remains “an eternal optimist” and will continue to push for a durable cease-fire. He also said that, despite repeated failures, he continues to believe an Israeli-Palestinian deal is possible — and that Arab governments want it now more than ever.

But he also acknowledged that the United States has outstanding issues with Iran, not least the conflict in Syria, where Iran has been a key supporter of President Bashar al-Assad.

Although Kerry is often mocked for such hopeful beliefs — critics see them as naivete — he said his experiences growing up playing “in the bunkers and tunnels” of postwar Europe and then serving in “an avoidable war” had taught him that diplomacy is almost always preferable to the alternative.

“Trying to avoid conflict isn’t a weakness,” he said. “It is strength.”

Morello reported from Washington.

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