This Holocaust Remembrance Day, it's worth noting an inconsistency present in public statements about atrocity prevention. As scholars Gerald Caplan, Samuel Totten and Amanda Grzyb pointed out recently, while U.S. presidents solemnly declare that "never again" will the world stand by and allow genocide to happen, they continue to stand by and allow genocide to happen.

There's no reason to think this will change. Nor is it clear that it should.

While "never again" is a nice sentiment, it's not a useful public policy. It's seldom clear when the line has been crossed from an ordinary civil war into genocide. Even when it is, what precisely the United States and the international community ought do about it is hardly obvious, especially in the moment.

The cry is, of course, a reaction to the Holocaust. But neither the United States nor any of its allies fought World War II in response to that horror; indeed, it's almost unfathomable that the citizenry would have considered that a cause worthy of 200,000-plus dead, many times that wounded, and a massive economic undertaking.

None of the atrocities pointed to by Caplan, Totten and Grzyb come close to the mindboggling horror of rounding up and killing more than six million innocent men, women and children from around Europe to "purify" that race. Still, they're outrageous in their own right. Yet, in each instance, it's easy to see why American presidents decided not to go to war to stop them.

In 1979, having spent the previous three years doing nothing to stop the Cambodian genocide, Jimmy Carter swore that "never again will the world stand silent... fail to act in time to prevent this terrible act of genocide."

Carter was, more than any president in my lifetime, staunchly antiwar. He came by it honestly, as a Naval Academy graduate with a distinguished career in the Navy before leaving to run the family business. But even if he had been a hawk, there was simply no way that he could have led the country into war in Cambodia so soon after we had finally extracted ourselves from the Vietnam fiasco.

And then Ronald Reagan: "I say in a forthright voice, Never Again!" Yet he was an enthusiastic backer of Guatemalan president Rios Montt, now on trial for genocide against his own people, while renewing the U.S. alliance with Iraq's Saddam Hussein even while Saddam was gassing the people of Halabja - the precise genocidal crime for which he was to be tried, had he lived.

The backing of Montt, who came to power in a coup and lasted a year and a half before suffering the same fate, is hardly surprising. Brutal regimes were hardly unique in Guatemalan history and he backed the Reagan administration's anti-Communist efforts in the region. And, of course, Hussein was the enemy of our enemy, Ayatollah Khomeini's Iran.

George H. W. Bush told the world that his visit to Auschwitz left him "with the determination not just to remember but also to act." Yet his silence as Serbia attempted to ethnically cleanse Bosnia was so thunderous that his rival in the 1992 campaign declared, "If the horrors of the Holocaust taught us anything, it is the high cost of remaining silent and paralyzed in the face of genocide."

The Yugoslav wars of the 1990s were incredibly messy, with several sub-national groups seeking to break away in a country where geography and ethnicity were, by the conscious design of the former communist regime, not aligned. While there was consensus in the West that Slobodan Miloševic's response to the crisis was brutal and criminal, the tragic reality was that the ethnic cleansing that made international intervention so compelling was ultimately the very thing that made effective action possible.

In Bosnia alone, there were multiple competing factions—the Yugoslav central government, the government of the Republic of Bosnia and Herzegovina, and the ethnic forces of Republika Srpska and Herzeg-Bosnia. While Miloševic was the pretty obvious bad guy, there were no good guys. Atrocities were commonplace and "ethnic cleansing" was a common policy of trying to remove people of the "wrong" ethnicity from a given area.

 . . . Bill Clinton went even further. Opening the Holocaust Memorial Museum in Washington, Mr. Clinton proclaimed that the U.S. had done too little to stop the Holocaust. "We must not permit that to happen again." That was April, 1993. Precisely one year later, for the crassest of partisan political reasons, his administration chose to allow perhaps a million Rwandan Tutsi to be slaughtered in one of the purest genocides on record.

The reasons were neither crass nor partisan. Clinton had massively cut the defense budget to achieve a post–Cold War "peace dividend," and the nation was still reeling from the "Black Hawk Down" fiasco in Somalia and weary of intervention in others' civil wars. While Clinton would later express extreme regret over having let the Rwanda tragedy happen, it's hard to imagine how he could have mustered the popular and Congressional support to do otherwise.

As for George W. Bush, he declared the Sudanese government guilty of committing genocide against the people of Darfur while working closely with that same government on his "War on Terror."

Having invaded first Afghanistan in response to the 9/11 attacks and then Iraq for a complicated myriad of reasons, the notion that Bush could have launched yet a third war—against a U.S. ally in the war on terrorism—boggles the mind.

Following these precedents, Mr. Obama's Atrocities Prevention Board seems to have accomplished little to nothing over the past twelve months. A completely secretive organism, it has not even issued any pronouncements in regard to the world's ongoing humanitarian crises, not least the Government of Sudan's daily bombings for the past year and a half against its own people in the Nuba mountains and in Blue Nile State.

In fairness to Obama, he did launch a major air campaign, in conjunction with our NATO Allies, in order to prevent a possible massacre in Benghazi. But the public appetite for international interventions is thin.

This doesn't necessarily mean that "never again" is mere lip service. There are actions short of military intervention that the United States and its allies and partners can take in response to atrocities. Indeed, many of the perpetrators of those pointed to by Caplan, Totten and Grzyb have been indicted for and faced trial for crimes against humanity. Sanctions and other diplomatic tools are also available.

The reality is and will continue to be that the United States will be inconsistent in its response to outrageous conduct. Intervening in Libya was relatively easy, whereas intervening in Syria—where the regime’s atrocities now dwarf those that got us into Libya—is too hard. Paradoxically, having intervened in Libya makes gaining consensus for doing so in Syria more difficult, as both China and Russia think NATO overstepped the mandate of Resolution 1973.

Ultimately, "never again" is an ideal rather than a red line. The international community has formed a consensus that the murder of innocents is unacceptable conduct marking regimes that engage in it as pariahs. Over the last two decades, for example, the Responsibility to Protect doctrine has slowly taken shape. But the conduct of international affairs remains messy, with states facing multiple, often conflicting interests and values. Strong states, alas, are afforded more leeway than weak ones.

James Joyner is managing editor of the Atlantic Council.