As American foreign policy continues its long string of failures—not a series of singles and doubles, as President Obamaasserted in a recent news conference, but rather season upon season of fouls and strikes—the question becomes: Why?

Why does the Economist magazine put a tethered eagle on its cover, with the plaintive question, "What would America fight for?" Why do Washington Post columnists sympathetic to the administration write pieces like one last week headlined, "Obama tends to create his own foreign policy headaches"?

As American foreign policy continues its long string of failures—not a series of singles and doubles, as President Obamaasserted in a recent news conference, but rather season upon season of fouls and strikes—the question becomes: Why?

Why does the Economist magazine put a tethered eagle on its cover, with the plaintive question, "What would America fight for?" Why do Washington Post columnists sympathetic to the administration write pieces like one last week headlined, "Obama tends to create his own foreign policy headaches"?

The administration would respond with complaints, some legitimate, about the difficulties of an intractable world. Then there are claims, more difficult to support, of steadily accumulating of minor successes; and whinges about the legacy of the Bush administration, gone but never forgotten in the collective memory of the National Security Council staff.

More dispassionate observers might pick out misjudgments about opportunities (the bewitching chimera of an Israeli-Palestinian peace, or the risible Russian reset), excessively hopeful misunderstandings of threats (al Qaeda, we were once told, is on the verge of strategic defeat), and a constipated decision-making apparatus centered in a White House often at war with the State and Defense departments.

There is a further explanation. Clues may be found in the president's selfie with the attractive Danish prime minister at the memorial service for Nelson Mandela in December; in State Department spokeswoman Jen Psaki in March cheerily holding up a sign with the Twitter hashtag #UnitedForUkraine while giving a thumbs up; or Michelle Obama looking glum last week, holding up another Twitter sign: #BringBackOurGirls. It can be found in the president's petulance in recently saying that if you do not support his (in)action in Ukraine you must want to go to war with Russia—when there are plenty of potentially effective steps available that stop well short of violence. It can be heard in the former NSC spokesman, Thomas Vietor, responding on May 1 to a question on Fox News about the deaths of an American ambassador and three other Americans with the line, "Dude, this was like two years ago."

Often, members of the Obama administration speak and, worse, think and act, like a bunch of teenagers. When officials roll their eyes at Vladimir Putin's seizure of Crimea with the line that this is "19th-century behavior," the tone is not that different from a disdainful remark about a hairstyle being "so 1980s." When administration members find themselves judged not on utopian aspirations or the purity of their motives—from offering "hope and change" to stopping global warming—but on their actual accomplishments, they turn sulky. As teenagers will, they throw a few taunts (the president last month said the GOP was offering economic policies that amount to a "stinkburger" or a "meanwich") and stomp off, refusing to exchange a civil word with those of opposing views.

In a searing memoir published in January, former Defense Secretary Robert Gates describes with disdain the trash talk about the Bush administration that characterized meetings in the Obama White House. Like self-obsessed teenagers, the staffers and their superiors seemed to forget that there were other people in the room who might take offense, or merely see the world differently. Teenagers expect to be judged by intentions and promise instead of by accomplishment, and their style can be encouraged by irresponsible adults (see: the Nobel Prize committee) who give awards for perkiness and promise rather than achievement.

If the United States today looks weak, hesitant and in retreat, it is in part because its leaders and their staff do not carry themselves like adults. They may be charming, bright and attractive; they may have the best of intentions; but they do not look serious. They act as though Twitter and clenched teeth or a pout could stop invasions or rescue kidnapped children in Nigeria. They do not sound as if, when saying that some outrage is "unacceptable" or that a dictator "must go," that they represent a government capable of doing something substantial—and, if necessary, violent—if its expectations are not met. And when reality, as it so often does, gets in the way—when, for example, the Syrian regime begins dousing its opponents with chlorine gas, as it has in recent weeks, despite solemn deals and red lines—the administration ignores it, hoping, as teenagers often do, that if they do not acknowledge a screw-up no one else will notice.

The Obama administration is not alone. The teenage temperament infects our politics on both sides of the aisle, not to mention our great universities and leading corporations. The old, adult virtues—gravitas, sobriety, perseverance and constancy—are the virtues that enabled America to stabilize a shattered world in the 1940s, preserve a perilous order despite the Cold War and navigate the conclusion of that conflict. These and other stoic qualities are worth rediscovering, because their dearth among our leaders is leading them, and us and large parts of the globe, into real danger.

Mr. Cohen was counselor of the State Department from 2007-08.