Teaching U.S. foreign policy at Stanford, I had been waiting for a piece like “The Obama Doctrine” for seven long years. Missing among reams of adulation and vituperation directed at the president was an intimate, yet dispassionate look like Jeffrey Goldberg’s that would help my students penetrate the most puzzling presidency since America’s entry into the great-power system, circa 1917.

Who is this Barack Obama, where does he fit into the traditional matrix of American thought on foreign policy: realism vs. idealism, isolation vs. intervention, power politics vs. liberal institutionalism? Above all, how to crack the mother of all mysteries: Is Obama overseeing the self-containment, or “self-disempowerment,” of the mightiest nation on earth?

Normally, starring powers are pushed off center-stage by more muscular players, as were the Habsburg Empire, France, and Britain in earlier centuries. Yet Obama’s America has been slinking off without duress. To sharpen the puzzle, Goldberg quotes the King of Jordan: “I think I believe in American power more than Obama does.”

Of course, it made sense back at the beginning of the Obama era, in 2009, to go for retrenchment instead of overreach. The U.S. was stuck in two endless wars while battling financial catastrophe. Afghanistan and Iraq were trillion-dollar power failures dramatizing America’s inability to buy order with brute force. Obama’s terse prescription was on target: “Don’t do stupid shit.”

So no more regime change or nation-building. Now what? “You could call me a realist in believing we can’t relieve all the world’s misery,” Obama muses to Goldberg. True enough. It is also true that realism is about the economy of power, which dictates the cold-eyed distinction between peripheral and “direct” threats, as Obama has it. Keep soaring ends in line with inherently limited means; that is another unassailable principle.

The devil is in the execution. In Obama’s mind, the Syrian Civil War does not constitute a direct threat; nor does Vladimir Putin’s lunge into Ukraine. For Obama, as Goldberg paraphrases No. 44, “the Middle East is no longer terribly important to American interests”; even if it were, “an American president could do little to make it a better place.” All told, in Goldberg’s words, Obama believes that the “the price of direct U.S. action would be higher than the price of inaction.”

Realism is more complicated. A realist knows that distant threats, if ignored, can turn into direct ones. Hence, the “precautionary principle”—better to act than wait in the face of risks not fully known—that is so dear to climate warriors like Obama serves as another pillar of the realist faith. A realist also knows that the international system, like nature, abhors a vacuum. So ambitious rivals will interpret inaction as invitation. Even that ur-isolationist Thomas Jefferson grasped the simplest rule of realism: Power calls for counter-power. “None of us wish to see Bonaparte conquer Russia,” he wrote in 1814. “This done, England would be but a breakfast. ... It cannot be to our interest that all Europe should be reduced to a single monarchy.”

The Romans had a word for it: principiis obsta, meaning “resist the beginnings” to avoid an unpleasant end. Syria is a perfect case study. Obama drew his vaunted “red line” over the use of chemical weapons in Syria before Bashar al-Assad massacred civilians with sarin, a nerve gas, in 2013. But instead of making true on the threat of an American military response, Obama pulled back and invited the Russians in, never mind that Henry Kissinger had essentially kicked them out of the Middle East in the 1970s—pushing them out of Egypt, Russia’s main stronghold in the region, by bringing then-President Anwar al-Sadat into the American camp. Mr. Putin was delighted to oblige Mr. Obama, and there went 40 years of American primacy in the world’s most critical arena.

Today, the Russians are back in force, posing a deadly risk to anybody who would dislodge them from the Levant. Today, Assad’s army is on a roll; millions of refugees have flooded into neighboring lands and into Europe.

When abandoning Iraq, the Obama administration pooh-poohed ISIS as a “flash in the pan.” Today, that “jayvee team” rules over a quasi-state athwart Syria and Iraq, from which it orchestrates mass murder in Paris. Thus, distant threats turn into direct ones, and so U.S. forces are returning to Iraq to fight an indecisive air war against the jayvees. Watch the Taliban regain power if the U.S. withdraws completely from Afghanistan.

This is what happens when U.S.-made vacuums beckon. Now, realists don’t have to fight every battle. The Brits did nicely as the “offshore balancer” who engineered the coalitions that laid low the hegemonist du jour, from Habsburg to Hitler. Yet Goldberg has not unearthed a Churchill in the Oval Office, but a “Little Englander” who would rather watch from afar than wade in.

“Yes, we can” has segued into “others will.” Who are they? Not the Europeans who ran out of ammo in the air war against Qaddafi’s Libya and then outsourced the job to the U.S. Air Force. Not the Iraqis who barely stopped ISIS at the gates of Baghdad. The Afghan army will not best the Taliban on its own. Nor can Saudi Arabia and the “Gulfies” take care of ISIS and Iran for the United States.

Yet Obama imagines that they will precisely because the U.S. won’t. “Free riders aggravate me,” he tells Goldberg, betraying a grievous misunderstanding of what it means to be the world’s No. 1. A measure of free-riding is a given whenever a very strong power consorts with a bunch of weaker ones. To recruit and sustain the coalition, Mr. Big will always have to invest more—recall Gary Cooper’s Marshal Kane, who begs, bribes, and bullies in order to round up a posse in the 1952 movie High Noon. Because he could not guarantee their safety, the good folks of Hadleyville passed the buck or defected to the bad guys.

Or followers turn away from the leader, as Saudi Arabia, America’s oldest ally in the Middle East, has done. Egypt, another mainstay, is sidling up to Russia again. Even Israel has struck a separate deal with Mr. Putin: We’ll let you prosecute your air war against America’s anti-Assad allies, if you don’t interfere with our attacks against Hezbollah’s arms pipeline from Iran to Lebanon.

No. 1, a.k.a. the “indispensable nation,” cannot go on vacation if it wants to keep the top-floor corner office—that is the moral of this tale. The British model—better to balance than to battle—is a treacherous guide for the 21st century. Britain could recruit from a pool of co-equal powers; the U.S. cannot. Who has comparable weight to America’s today? Russia, Iran, and China. Reaching out to them reflects another misreading of reality on Obama’s part. For they are not cohorts, but hard-core rivals who have been emboldened by America’s retraction.

You don’t have to be a wild-eyed neocon to understand that power talks. Nor do you have to grow up in gangland to know that street cred in the global arena depends on a reputation for violence that will render force unnecessary. If Obama were indeed a realist, he would appreciate such simple insights into the nature of men and nations. Not to put too fine a point on it: He is not a realist, but an isolationist with drones and special-operations forces.

To restate the puzzle: Where does the 44th president fit into America’s ideological history? The answer emerges at the end of “The Obama Doctrine.” His mindset is rooted in the oldest tradition of them all. Many of the Founders were isolationists with a special mission. Their lodestar was “not dominion, but liberty,” as John Quincy Adams had it. America would not go “abroad in search of monsters to destroy.” Nor would it have to; that was the best part. History would do the job, bringing about the foreordained triumph of freedom and democracy.

“Obama believes that history has sides”—this is how Goldberg sums up the president’s faith. “America’s adversaries—and some of its putative allies—have situated themselves on the wrong one, a place where tribalism, fundamentalism, sectarianism, and militarism still flourish. What they don’t understand is that history is bending in his direction.”

If history is on America’s side, then Americans need not force or fight others. Let Iran, Russia, and China push their pawns forward in search of glory and power. It does not matter, Obama believes, because they are destined to fail while America’s king will shine across the chessboard.

This is not grand strategy. It is religion. Yet the central myths of Judeo-Christianity are the Pharaonic Slavery and the Crucifixion. They warn that tragedy comes before redemption.