Correction: An earlier version of this column misstated the date of President Obama’s speech to the U.N. General Assembly. It was in September 2013, not October 2013. This version has been updated.

Last week President Obama said the United States should consider unilateral military action only when the nation or its allies are directly threatened.

His stance would repudiate decades of bipartisan foreign policy — and Obama’s own views of just 10 months ago.

What might have caused his shift? I have a theory. But first, look at how his views have evolved.

At West Point on Wednesday, the president said, “The United States will use military force, unilaterally if necessary, when our core interests demand it — when our people are threatened; when our livelihoods are at stake; when the security of our allies is in danger.”

But, he said, “when crises arise that stir our conscience or push the world in a more dangerous direction but do not directly threaten us . . . we should not go it alone. Instead, we must mobilize allies and partners to take collective action.”

Presidents generally seek allies for their foreign interventions. But the idea that the United States couldn’t act alone to stop a genocide, say, would have come as a surprise to previous presidents.

Bill Clinton wrote in his 2004 memoir that “the failure to try to stop Rwanda’s tragedies became one of the greatest regrets of my presidency.” Susan Rice, now Obama’s national security adviser, urged President Bush in 2006 to bomb Sudan to help save the people of Darfur, with or without United Nations approval. Would that violate international law? “Perhaps,” she wrote breezily in a Post op-ed, coauthored by Clinton’s national security adviser Anthony Lake and one other writer, but it might be sanctioned by “a new international norm prescribing ‘the responsibility to protect.’ ”

And in August, after Bashar al-Assad gassed 1,400 of his Syrian countrymen to death, Obama urged Congress to authorize unilateral force. “I’m comfortable going forward without the approval of a United Nations Security Council,” the president said.

He framed the challenge as one of national security, but not because the United States or its allies were under attack. No, this was a crisis that stirred our conscience and pushed the world in a more dangerous direction.

“What message will we send if a dictator can gas hundreds of children to death in plain sight and pay no price?” Obama asked. “We cannot raise our children in a world where we will not follow through on the things we say, the accords we sign, the values that define us.”

The threat of bombing persuaded Assad to agree to give up his chemical arsenal, and 10 months later, most of his declared weapons, at least, have been turned over for destruction.

Obama claims the outcome as a significant accomplishment. Yet he now rejects the logic that brought it about — the premise that America may threaten force to protect “the values that define us.” Why?

Here’s one possibility. Whatever the success of his chemical weapons deal — debatable now that Assad is dropping undeclared chlorine gas bombs — Obama’s overall Syria policy is a failure by any measure. While he has resisted advice, over three years, to robustly assist Assad’s moderate opposition, the country has been decimated: More than 50,000 civilians killed, hundreds systematically tortured, millions displaced. Al-Qaeda terrorists control swaths of Syrian territory, where American and European jihadists train to attack their homelands.

Syria has become, in the words of Obama’s U.N. ambassador Samantha Power, “the most catastrophic humanitarian crisis any of us have seen in a generation.”

That has to be painful for Obama to confront. As the situation has worsened, he has offered many rationales for why aloofness was the right policy — and he has begun to retroactively rewrite U.S. doctrine to explain why intervention was impossible.

In September, in a speech to the U.N. General Assembly, Obama limited America’s “core interests” in the Mideast to defending allies, fighting terrorism and weapons of mass destruction, and protecting the free flow of energy. He said democracy, human rights and free trade were important, but not core, and could “rarely” be advanced through unilateral action.

Last week he further constricted the nation’s core interests and closed the narrow opening he had left for action in defense of values; the “rarely” was gone.

I doubt this is really where Obama wants to end up. A values-free foreign policy isn’t sustainable for most Americans, and if another humanitarian crisis confronts the president in the next 30 months, I doubt it will be sustainable for him.

Instead, he’s likely to be swayed by arguments like this: “Terrible things happen across the globe, and it is beyond our means to right every wrong. But when, with modest effort and risk, we can stop children from being gassed to death, and thereby make our own children safer over the long run, I believe we should act. That’s what makes America different. That’s what makes us exceptional. With humility, but with resolve, let us never lose sight of that essential truth.”

That was Obama speaking, of course. Sept. 10, 2013.

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