A Turkish armored vehicle patrols at the Turkish-Syrian border area opposite the Syrian town of Ain al-Arab, known as Kobane by the Kurds, in the southeastern village of Mursitpinar, in Sanliurfa province, on October 13, 2014. (Aris Messinis/AFP/Getty Images)

The value of U.S. foreign policy conducted by majority vote — which might have resulted in a Nazi-occupied London — is once again evident.

In 2013, 52 percent of Americans agreed that their country should “mind its own business internationally.” (In 1964, the figure was 20 percent.) This robust consensus for disengagement was soon followed by the rapid expansion of the Islamic State in a vacuum left by U.S. inattention, and then by an outbreak of Ebola in West Africa that should have been confronted months earlier with larger resources.

In recent years, Americans have generally gotten what they wanted on foreign policy issues — and they now ruthlessly punish those who implemented their will. A recent poll has President Obama’s foreign policy approval rating at a sorry 34 percent. Americans may applaud “nation-building at home,” but they eventually make judgments based on outcomes in Mosul and Monrovia. And this is fair. A commander in chief does not sign on to reflect public consensus but to defend the country and the Constitution.

This interplay between an often-reluctant public and a chief executive energetically pursuing the national interest has generally served the country well. Presidents since World War II have possessed broad powers to exercise American influence on a global stage — declaring strategic doctrines, enforcing “red lines,” arming proxies, sending emergency assistance, striking enemies with drones — while trying to persuade Congress to fund such efforts and Americans to support them.

This is a different role for a U.S. president than in Jefferson’s old republic. But it emerged when it did for a reason. German philosopher Karl Jaspers argued that history before World War II had been “a dispersed field of unconnected ventures.” With the war, it became “the totality, which has become the problem and the task.” The world, for the first time, had a meaningfully unified history. A variety of challenges — nuclear proliferation, environmental degradation, terrorism — eventually became global in nature. And some old problems, including pandemic disease, gained the speed of air travel and the momentum of growing, tightly packed populations.

Since World War II, the United States has been the only nation willing enough, capable enough and responsible enough to keep a semblance of order. The divided states of Europe will sometimes join a coalition but seldom lead it. Russia and China remain engaged in Great Power games. International institutions are weak at best. The United States has consistently encouraged the capabilities of other responsible actors. But it is still the U.S. 7th Fleet that deploys near Japan; the 2nd Infantry Division that stands guard at the Korean Demilitarized Zone; the U.S. Central Command that conducts most of the airstrikes in Iraq and Syria.

Some naturally complain about these burdens. But without the United States bearing them, the horrors of the 20th century would have been infinitely more horrible. The death camps emptied only by ruthless efficiency. Or Europe unified under Soviet rule, with gulags outside Berlin and Paris.

Threats to the United States have shifted — or perhaps diversified. The 9/11 attacks originated in the failed and distant state of Afghanistan. Afterward, some academics argued that this case was unique and that the “problem of failed states” was exaggerated. Now the Islamic State feeds off governing failure across two countries. And Ebola spreads in the absence of effective health-care structures across three countries. So the United States has no real interest in the problem of failed states — except when it results in al-Qaeda, the Islamic State and Ebola.

President Obama’s embrace of these duties has been two-sided. He is, intellectually, an internationalist who speaks of the dangers of globalized threats. But he ran for office promising national retrenchment in order to focus on domestic concerns, and he has cited the polls (on Syria in particular) as confirmation of his own instinct for inaction. His prosecution of war against the Islamic State seems (so far) constrained and half-hearted.

Some conservatives are no better — imagining that global problems can be contained by the isolation of Syrian chaos (“baddies vs. baddies” anyway), or by the effective quarantine of Liberia.

But 200,000 dead in Syria, along with 9 million displaced, proved to be an uncontainable regional catastrophe. And the isolation of Ebola-affected nations could accelerate economic and political collapse, increase suffering and death, and result in further spread of the disease. It will not work — and it cannot be right — to allow these countries to die behind a curtain.

The United States has problems that can’t be isolated, only confronted. And the longer it takes to realize it, the harder our tasks become.

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