Furious Republican opposition to a deal over Iran’s nuclear program may look like another example of political partisanship and personal animosity toward Barack Obama. But there’s also a much deeper reason for congressional pushback: the deeply ingrained aversion in American culture toward parleying with 'evil' opponents.

Negotiating with international adversaries is more controversial in the United States than in most advanced democracies. Whereas in other countries bargaining is often seen as the norm, Americans frequently view face-to-face talks as a prize that the opponent has to earn through good behavior. The United States is part of a coalition of six countries talking to Iran, alongside Britain, France, Russia, China, and Germany. But the United States is the only country where the deal has generated a domestic political storm.

Resistance to an agreement with Iran is part of a long history of American skepticism about engaging with adversaries. During the early years of the Vietnam War, France, Britain, Canada, and the UN secretary general all entreated the United States to negotiate an end to the conflict—but Washington refused to entertain any settlement short of North Vietnam’s surrender.

More recently, in the 1990s, Republicans assailed President Bill Clinton’s efforts to negotiate a nuclear deal with North Korea, with Senator John McCain calling the process tantamount to “appeasement.”

In 2003, when Iraq began spiraling into chaos, Deputy Secretary of Defense Paul Wolfowitz received a memo suggesting that the United States engage disaffected Sunnis. He reportedly returned it with three words scribbled in the margin: “They are Nazis!”

In 2008, President George W. Bush told the Israeli Knesset, “Some seem to believe that we should negotiate with the terrorists and radicals, as if some ingenious argument will persuade them they have been wrong all along.” But this contention, he said, was only “the false comfort of appeasement.”

For a decade after the United States invaded Afghanistan, the Bush and Obama administrations refused to engage the Taliban insurgents. A major review of U.S. strategy in 2009 didn’t even consider the option of a diplomatic solution with the Taliban leadership. In 2010, U.S. Special Representative for Afghanistan Richard Holbrooke asked General David Petraeus about Afghan reconciliation. “Richard, that’s a 15-second conversation,” Petraeus reportedly responded. “Yes, eventually. But no. Not now.” Later, during the 2012 presidential campaign, the Republican presidential candidate Mitt Romney declared: “We should not negotiate with the Taliban, we should defeat the Taliban.”

Resistance to diplomatic engagement is usually strongest on the political right. In 2013, for example, 27 percent of Democrats disapproved of an interim agreement with Iran on its nuclear program—compared with 58 percent of Republicans. Democratic presidents are particularly vulnerable to attack as “appeasers” for talking to enemies. By contrast, it’s often politically easier for hawkish Republicans to negotiate because few doubt their toughness. During the Cold War, Democratic presidents started the big wars in Korea and Vietnam. And Republican presidents talked America’s way out of them. As the saying goes, it took a Nixon to go to China.

Why do Americans dislike negotiating with enemies? The answer has to do with the country’s combination of power and moralism. After 1945, the United States bestrode the world like a colossus, with an economy three times larger than that of its nearest competitor, the Soviet Union. America’s globe-girdling capabilities diminished the apparent need to compromise with weaker opponents like North Vietnam, which Lyndon Johnson called a “raggedy-ass little fourth-rate country.”

Meanwhile, according to a moralistic strain in American culture, compromising with 'evil' opponents sullies U.S. values. Americans tend to be deeply committed to the nation’s ideals of democracy and individual rights. Rates of religiosity are also far higher in the United States than in other rich democracies. As a result, Americans are more likely to say there are absolute standards for good and evil than Europeans, Canadians, and Japanese, who are more likely to say that ethics depend on circumstances.

American moral righteousness can make the act of bargaining seem inherently suspect. “Because we fight for values and we fight for principles,” said LBJ about Vietnam, “our patience and our determination are unending.”

Sometimes it’s appropriate not to engage enemies—particularly when they’re utterly extreme or intransigent, like the Nazis or al-Qaeda. But these are rare exceptions.

The danger is that American power and moralism triggers opposition to diplomacy that undermines U.S. interests. For all its vast resources, the United States can’t usually enforce its will against enemies—especially when a dispute is in the opponent’s backyard. And refusing to compromise American values by making concessions can mean paying a larger ethical price later on. Washington may miss opportunities for a valuable deal and end up negotiating as a last resort—when its hand is much weaker.

The United States should have engaged the Taliban in 2002 or 2003. At the time, the insurgents were on the run and might have offered major concessions in return for a recognized political role in the new Afghanistan. After all, the Taliban were weakened but not defeated. They enjoyed sanctuaries in Pakistan. There were only a few thousand international troops in Afghanistan. An insurgent comeback was always a possibility. So why not negotiate when the United States still held the best cards? Instead, Washington waited until 2011, when the war had deteriorated into a quagmire, to publicly endorse peace talks for the first time.

Similarly, the United States avoided meaningful negotiations with Iran when Tehran had a few hundred centrifuges—holding back until the Iranians had constructed an industrial-scale nuclear program.

“Many consider negotiations as a sign of weakness,” wrote Henry Kissinger. “I always looked at them as a weapon for seizing the moral and psychological high ground. ... It is a device to improve one’s strategic position.”

To employ this diplomatic weapon effectively, Obama must maneuver through a highly challenging political landscape. For a Democratic president, the ideal solution is to build a bipartisan foreign-policy team. This is no easy feat given the current hyper-partisan mood. But Obama pulled off this trick once with the appointment of Robert Gates, a moderate Republican, as secretary of defense. The two Iranian nuclear crises—the one in the Middle East and the one in Washington, D.C.—show that Democrats have a tough time making peace on their own. In the end, it might take a Republican to go to Iran.