Last Thursday, Ed Henry of Fox News asked White House press secretary Josh Earnest a straightforward question about President Obama's decision to authorize force against the Islamic State: "Is preventing a genocide in America's core interests?"

Earnest paused, then paid lip service to one of the most persistent truisms in American foreign policy: "Of course the United States has been and will continue to be a beacon for freedom and respect for basic human rights around the globe. And that is a core founding principle of this country and one that American men and women have fought and died to protect."

The current generation seems to believe that preventing genocide around the world has always been in the United States' interest. But Washington has had a dismal record of stopping genocides and ethnic cleansing, and that is unlikely to change.

Probably the first example was in the 1880s, when President Chester A. Arthur recognized and supported Belgian King Leopold's claims to the Congo. Leopold's brutal rule — indiscriminate violence against local populations, collective punishment and mutilations — led to the death of several million Congolese. Despite decades of lobbying for the United States to take a strong position against Leopold, Washington remained reluctant. Teddy Roosevelt, president from 1901 to 1909, said "it was a literal physical impossibility to interfere."

In subsequent decades, U.S. presidents more isolationist than Roosevelt refused to stop Japanese atrocities in East Asia, Turkey's genocide in Armenia or European colonizers' large-scale killing of civilians in places like Southeast Asia. Josef Stalin's forced deportation of some 6 million minorities in the Soviet Union in the 1930s — ethnic cleansing in its truest sense — did not diminish the admiration for him by some in the highest levels of U.S. government.

What about the Holocaust? U.S. war efforts certainly contributed to the defeat of Nazi Germany and put an end to the most horrifyingly industrialized genocide in history. But American popular lore often overlooks the fact that on Dec. 11, 1941, Adolf Hitler first declared war on the United States — and not vice versa. The United States' humanitarian intentions — despite having learned about Auschwitz and other concentration camps — were an afterthought.

When the Khmer Rouge conducted its reign of terror in Cambodia from 1975 to 1979, leading to the death of up to 25 percent of its roughly 7 million people — proportionally the largest genocide of the 20th century — Washington remained aloof. Because of its geopolitical interests of opening up to China and spurning the Soviet Union and Vietnam, it opted for a policy of nonintervention. The United States was even critical of the 1978-79 invasion of Cambodia by a pro-Soviet Vietnam that ended Pol Pot's reign.

Similarly, when Saddam Hussein used chemical and conventional weapons to kill an estimated 100,000 ethnic Kurds in Iraq in 1988, Washington — having recently made overtures to Baghdad, with which it then had a common adversary in Iran — did not even impose sanctions.

It was the hands-off approach to the genocide in Rwanda in 1994, at a moment of unchallenged U.S. global supremacy, that awoke the United States from its slumber. Many, including current U.S. National Security Adviser Susan Rice, then a young National Security Council official dealing with international organizations and peacekeeping, felt that Washington could and should have stopped the genocide, which saw members of the Hutu ethnicity slaughter more than half a million ethnic Tutsis in just a few months.

Given a century-long track record of nonintervention, the 1999 U.S.-led bombing campaign in Yugoslavia to stop the Serbian ethnic cleansing of Kosovar Albanians was an aberration. But even in that case, Washington was arguably influenced as much by other considerations as by altruism. And America's subsequent record — failing to stop the ethnic cleansing in the Sudanese region of Darfur, in the shambolic Central African Republic, or (again) in the Congo, has been dismal.

Why then do so many Americans cling to the belief that genocide prevention has been — or could be — a core national interest? Some of the delusion may stem from America's self-image as a moral superpower, combined with the unambiguous success of the 1999 NATO bombing campaign in the Balkans. And it is hard to underestimate the influence of the 2002 book "A Problem from Hell" by Samantha Power — now U.S. ambassador to the United Nations — on the foreign-policy community.

None of this grim history should mean that the United States lacks a moral compass in its international relations. Nor does it mean that Washington should not help the Yazidis and other minority groups at the receiving end of the Islamic State's savagery. That is a call for senior U.S. leaders to make, taking into account their country's interests, abilities, costs and risks. But let's get one thing straight: Stopping genocide is not a core U.S. national security interest, nor has it ever been, and realizing this would be better than radiating false hope to persecuted minorities the world over.

Dhruva Jaishankar is a fellow with the German Marshall Fund in Washington.