The Jungle Grows Back: America and Our Imperilled World. By Robert Kagan.Knopf; 192 pages; $22.95.

IT IS a common belief that human rights and democracy must prevail because they are noble and good. The world’s benighted multitudes yearn for freedom and one day will win it. Robert Kagan, a historian, thinks that is a dangerous fantasy.

Though he rejects the label, Mr Kagan is a neocon—an unfashionable proponent of using force to support liberal values. As he says at the start of his new book: “What we liberals call progress has been made possible by the protection afforded liberalism…by American power.” The question is what happens to freedom when that power retreats? His title captures his alarming answer: “The Jungle Grows Back”.

Mr Kagan touched on this theme in “The World America Made”, published in 2012, which explained the spread of democracy after the second world war in terms of growing American hegemony. But that was before the election of President Donald Trump—a commander-in-chief who has not so much neglected foreign policy as given it a good kicking.

Although Mr Trump rarely appears in Mr Kagan’s latest book by name, it is a devastating riposte to his careless, cynical and destructive approach to diplomacy. Mr Kagan tells of the chaos after the first world war and the rise of Bolshevism and fascism it engendered. He explains how the second world war convinced Americans that “their way of life could not be safe in a world where Europe and Asia were dominated by hostile autocratic powers.”

Americans have forgotten that any hostile autocracy is a threat—not just the Soviet Union (which, incidentally, was not seen as an inevitable foe until well after America’s decision to abandon isolationism). Although containment of the Soviet Union became the focus of American policy during the cold war, the underlying logic for global engagement has survived the Soviet collapse. And yet there have been growing calls for America to stop acting as the world’s policeman. After the invasion of Iraq, and amid the long war in Afghanistan, those calls became almost impossible for politicians to resist.

Under Mr Trump, Mr Kagan argues, Americans have also forgotten that their allies will submit to a Pax Americana only if it seems fair. If an alliance is merely a vehicle for American power, if it neglects the security of its junior partners, they will drift away. If America adopts a Hobbesian vision of a brutal, zero-sum world, as Mr Trump seems to, everyone else will, too.

Where does this analysis lead? “World order is one of those things people don’t think about until it is gone,” Mr Kagan warns. People take for granted how America’s security guarantees have prevented the merciless escalation of a dog-eat-dog world. Post-war Germany and Japan felt safe enough to forsake militarism and concentrate on economic growth—as they had not in the late 19th and early 20th centuries. Even the Soviet Union trusted America enough to give up without a fight.

A common belief holds that world peace can sustain itself. In fact, “vines and weeds…are constantly working to undermine it”. Mr Kagan thinks that without America’s management, the world faces what the State Department in 1945 called “power politics pure and simple”. Unstable spheres of influence and mutual suspicion will ensue. “We do not face a choice between good and bad but between bad and worse,” he writes. “It is between maintaining the liberal world order, with all the moral and material costs that entails, or letting it collapse and courting the catastrophes that must inevitably follow.”

This is a bleak vision. Even if Mr Kagan underestimates the power of an idea to stir people—as liberal democracy did in eastern Europe during the cold war—he is right to detect a crisis of confidence in the democratic world. He sets out his case with characteristic brilliance and conviction. The tragedy is that, in Mr Trump’s America, his words are almost certain to be scorned.