Secretary of State John Kerry, right, meets with Russian Foreign Minister Sergey Lavrov in Geneva in March. (Evan Vucci/Associated Press)

An earlier version of this column did not disclose that Stephen F. Cohen is Katrina vanden Heuvel’s husband. The column has been updated.

The nuclear agreement with Iran provides ample proof of Secretary of State John F. Kerry’s remarkable commitment and skill in waging diplomacy. In an era when the Pentagon dominates our foreign policy and military options are too often trotted out as first responses, he has resuscitated the United States’ power to lead, pressure and negotiate, a capacity too often denigrated as “soft power.”

No good deed goes unpunished. His reward for this is not only a pitched battle at home with hawks in both parties intent on torpedoing the Iran deal, but also what will be an even fiercer struggle with higher stakes: fending off those intent on escalating a face-off with Russia over Ukraine into a new Cold War. Once more, Kerry must preserve our real security interests from those recklessly brandishing America’s military prowess.

The voices calling for a new Cold War — or worse — with Russia are growing louder. "If you want to talk about a nation that could pose an existential threat to the United States, I'd have to point to Russia," said Gen. Joseph F. Dunford Jr., at Senate hearings on his nomination to be the new chairman of the Joint Chiefs of Staff. Dunford listed Russia as "the greatest threat" to U.S. national security, which will surely startle those old enough to remember 9/11. Even as the United States announced that it was considering training Ukraine's regular forces, Dunford recommended shipping U.S. weapons to the Kiev regime, another step toward a proxy war with Russia that President Obama has thus far resisted.

Dunford was serving up red meat to what might best be termed the war party, a claque with adherents in Congress, the administration and the military, led by Sen. John McCain (R-Ariz.), Defense Secretary Ashton Carter, Assistant Secretary of State Victoria Nuland and NATO Supreme Allied Commander Philip Breedlove. The war party has conjured up a Russian threat to Europe and pushed the president into ever more belligerent postures toward Moscow, with NATO launching military exercises and stationing, for the first time, tanks, troops and ordnance on Russia’s western border.

Dunford deemed as "reasonable" the arming of Ukrainian forces — now notoriously revealed to include neo-Nazi "irregulars." But this would, as James Carden writes , virtually ensure an escalating conflagration, increase civilian casualties, upend the Minsk accords and quite likely provoke further Russian operations in the eastern part of the country.

Although focused on the Iranian negotiations, Kerry has tried sporadically to bring some light to all of this heat. State Department spokesman Mark Toner, responding to the Dunford testimony, told reporters: "Certainly we have disagreements with Russia . . . but we don't view it as an existential threat."

An ongoing Ukrainian civil conflict is rapidly being transformed into a NATO-Russia proxy war. Diplomatic exchange is supplanted by weapons deployment and military maneuvers. Cooperative relationships — from trade to arms control — are being dismantled. And the clamor for greater involvement drowns out any sensible assessment of our interests, the nature of the conflict within Ukraine and the reality of the increasingly authoritarian Kiev regime. The war party appears intent on turning all of Europe, up to the Russian border, into a U.S./NATO sphere of influence. The broader goal, trumpeted by some, is regime change in Russia itself.

That is folly. Russia may be an authoritarian state, but it does have nuclear weapons and a deep national pride. We have every reason, as Stephen F. Cohen has argued, to seek a new detente with Russia. (Disclosure: Cohen is this columnist's husband.) Cohen is a founding board member of the American Committee for East-West Accord, whose members also include former senator Bill Bradley and Jack Matlock, former ambassador to the Soviet Union under Ronald Reagan and George H.W. Bush. The committee believes that U.S. interests are best served by integrating Russia into a peaceful Europe, not re-creating separate and hostile camps. The United States should want to ease tensions, continue to reduce nuclear arsenals and cooperate internationally. It should want to provide space for reformers, not reinforce the hard-liners inside the Kremlin. As the Iran deal, as well as the effort to rid Syria's Bashar al-Assad of chemical weapons, reveal, real U.S. national security interests — from nuclear proliferation and international terrorism to the Middle East and Afghanistan — are best served with a partner, not an adversary in Moscow.

Kerry is an old Washington hand. He knows how threats are inflated and military measures can escalate. He experienced personally the folly of misbegotten war. When the seven foreign ministers who negotiated the Iran deal met for the last time, each spoke briefly about the moment. Speaking last, Kerry recalled going off to Vietnam as a young naval officer. He returned committed, he said choking up, to using diplomacy to avoid the horrors of war.

Now he must act to counter the war party inside the administration and Congress and revive diplomacy to forestall an escalating conflict with Russia. That means implementing the Minsk accords that have been undermined on all sides. It means creating a process that can lead to an agreement about Ukraine , guaranteeing its independence territorially, politically and militarily, and providing real home rule for regions historically aligned with Russia or with the West. None of that is possible without recognizing that Russia has legitimate concerns about its neighbor, just as the United States has about Mexico. None of that is possible without a diplomatic offensive that is powerful enough to counter the currents now feeding the conflict.

Although the war party continues to howl at the moon, Kerry has forged an agreement that will keep Iran from developing the bomb for a decade or more. Now, even as he defends that agreement, he must wage diplomacy in an area where the stakes are even bigger.

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To read more on this topic:

Henry A. Kissinger: Start at the end of the Ukraine crisis

Vladimir V. Kara-Murza: Ukraine is Putin’s war

Fiona Hill and Clifford Gaddy: Pushing Putin into a regional war

The Post’s View: Ukraine cease-fire does little to restrain Putin