It was a petty gesture, a stubborn mistake he still regrets. In 1997, after years of bickering and violence in Northern Ireland, British Prime Minister Tony Blair and his chief of staff, Jonathan Powell, met with leaders of the Irish Republican Army about a possible peace agreement. But when Powell sat down with his counterparts in the IRA—a group that shot and wounded his father and threatened to kill his brother—he refused to shake their hands.

A year later, the British and the IRA signed the Good Friday Agreement, ending the bitter conflict. Today, Powell, one of the deal’s main architects, looks back on it as his most difficult challenge and his most satisfying achievement. Speaking to Newsweek in his office in London, the 58-year-old says he once favored big sticks over speaking softly. But his experience with the IRA convinced him that opening a dialogue with terror groups is critical to defeating them, or at least to resolving conflicts.

In his new book, Terrorists at the Table: Why Negotiating Is the Only Way to Peace, Powell argues that terrorist organizations with significant popular support can’t be defeated by force alone, even if the West resorts to brutal and unscrupulous measures. That goes for nationalist groups like the IRA as well as religious extremists like ISIS and Al-Qaeda. “Each time we meet a new terrorist group, we say they are different and that we are never going to talk to them,” Powell says. “[But] anyone who thinks a bombing campaign alone can degrade and destroy ISIS is wrong. The sensible thing to do is to open a channel…so we can better understand each other.”

Today, Washington is embroiled in a seemingly endless war against ISIS and Al-Qaeda. And while the U.S. pulled its troops out of Afghanistan in 2014, it’s still trying to help the government in Kabul find a peaceful end to the war against the Taliban. In other words, the insights Powell offers have perhaps never been more relevant. “The problem is not talking to terrorists,” he writes, “it is giving in to them. They are not the same thing.”

For decades, America’s official policy has been to destroy terrorist organizations, not sit down with them for a beer summit or a cup of tea. President Ronald Reagan forged this policy in the early 1980s, not long after Iranian students stormed the U.S. Embassy in Tehran, taking 52 Americans hostage. “I believe it is high time that the civilized countries of the world made it plain that there is no room worldwide for terrorism,” Reagan said during a 1980 presidential debate with Jimmy Carter. “There will be no negotiation with terrorists of any kind.”

ISIS has made great inroads into Iraq and controls large swaths of Syria, only the Kurdish forces of Northern Iraq have dealt any major blows to the militant group, taking back territory such as this building in Hasaka, Syria on March 1, 2015. Rodi Said/Reuters

Yet behind the scenes or through proxies, the U.S. government has negotiated with terror groups. In the mid-’80s, the Reagan administration secretly sold weapons to Iran, violating an arms embargo, in hopes of freeing hostages in Lebanon held by a group with connections to Tehran. (Washington then used the proceeds to illegally fund anti-Communist rebels in Nicaragua.) In the early 1990s, President George H.W. Bush’s White House negotiated with Hezbollah to release American hostages in Lebanon. Years later, President Bill Clinton sent diplomats to talk to the Taliban and personally met with the leader of Sinn Féin, the political arm of the IRA. In 2002, not long after the war on terror began, George W. Bush’s administration organized an indirect payment of $300,000 for the release of two American missionaries held by Abu Sayyaf, an Islamist group in the Philippines.

Critics have long decried this policy, saying it creates confusion. In 2014, after ISIS kidnapped several Americans in Syria, the White House declined to pay ransom. It also told the families of the hostages that U.S. law barred them from negotiating with the terrorists. At the same time, however, the administration negotiated with the Taliban via Qatari intermediaries to free Army Sergeant Bowe Bergdahl in 2014.

Some advocate a harder line, fearing that talk and negotiation legitimizes violent fanatics. “It’s naive and dangerous to believe you can open a dialogue with Middle Eastern terrorist groups like Hamas, ISIS or Hezbollah,” says Nitsana Darshan-Leitner, head of the Shurat HaDin Israel Law Center, a civil rights organization that combats terrorist organizations. “In this region of the world, any act of political accommodation is perceived as weakness and just further spurs on more extremist violence.”

Powell, however, says it’s naive to rely on force alone to defeat terrorism. He points to the 1980s, when Hafez Assad, the father of Syrian strongman Bashar Assad, crushed a rebellion by Syria’s Sunni Muslim Brotherhood in the northern city of Hama. The elder Assad ringed the city with tanks and artillery and flattened it, killing tens of thousands of people. For all his brutality, however, Assad only temporarily buried the conflict. For years, bitterness stewed under the surface, erupting in the Arab Spring protests in 2011. Today, as the country’s civil war enters its fifth year, the fighting has killed more than 230,000 people and displaced millions more.

“Negotiating with terrorists is not a question of forgiving or forgetting the past,” Powell writes. “Holding a pragmatic position of the future…[places] the cessation of bloodshed as the highest priority.”

Just talking to terrorists, however, is as ineffective as trying to blast them all to smithereens, Powell says. Having left government, he now runs Inter Mediate, a nongovernmental organization that works to moderate conflicts. He won’t say exactly which conflicts he’s trying to resolve, but he still considers force a critical part of fighting terrorists and bringing them to the negotiating table—even fanatics like ISIS. “I wouldn’t talk to [ISIS] about a caliphate, about what they are demanding,” he says. “But…we need to start to understand them. Someone needs to go there and sit down with them to discuss: How do you view the treatment of Sunnis in Iraq and Syria?”

“Technically, anyone can be negotiated with at some point,” says Vali Nasr, dean of Johns Hopkins University’s Paul H. Nitze School of Advanced International Studies. “The issue is that these kind of insurgent groups or terrorist organizations are not always ready to talk. ISIS is on the rise. It’s triumphant. [The group] has to be humiliated before it will be prepared to settle for less than it thinks it can gain by fighting.”

Yet in the long term, Powell and others say, no group remains completely intransigent. “Not talking to people whom we designate as terrorists is a way of depriving ourselves of information about them, and ensuring we confirm our own pre-existing biases,” says Barnett Rubin, a former State Department official and a fellow at New York University’s Center on International Cooperation.

“Too many equate talking with appeasement,” says Peter Galbraith, a former U.S. ambassador to Croatia. “Talking is not giving anything away. It is hard to see how to talk to Al-Qaeda or [ISIS], given that they seem to be mostly about violence. But there may be circumstances where even that makes sense.”

What’s key, Powell says, is trust and timing. Historically, he argues, the distance between Washington’s official policies and its unofficial ones has been a hindrance to both. In 2004, ISIS’s precursor, Al-Qaeda in Iraq, rallied disgruntled Sunnis against the U.S. occupation. The group used suicide bombers and improvised explosive devices to attack Shiites and American troops. The U.S. apparently didn’t negotiate with the group’s leader, Abu Musab Zarqawi, but it did make deals with Sunni tribes that initially supported the insurgency but soon grew tired of Al-Qaeda’s brutality. This effort, known as the Sunni Awakening, helped drive Zarqawi’s group out of the country.

Powell says this effort should have started much earlier. “We usually delay talking to armed groups too long, and as a result, a large number of people die unnecessarily,” Powell writes in The Guardian. “General David Petraeus admitted that, in Iraq, the U.S. left it far too late to talk to those ‘with American blood on their hands.’ We delay because it is argued that talking is too risky—but experience suggests the real risk lies in not talking.”

Today, as a similar process seems to be playing out in Syria, the White House appears open to the idea of talking things out. Last month, President Barack Obama said the U.S. won’t make concessions to terror groups but is willing to talk to them. The government, he added, won’t prosecute the families of captives who are trying to do the same. For Powell, that means if the U.S. and other Western countries haven’t already started talking to ISIS and Al-Qaeda, sooner or later they likely will. As Hugh Gaitskell, the former British Labour Party leader, once put it, “All terrorists, at the invitation of the government, end up with drinks in the Dorchester.”

With Jonathan Broder in Washington, D.C.