In December 1940, as Nazi Germany menaced Europe, Franklin D Roosevelt declared the United States to be the “arsenal of democracy”. There was nothing inevitable about this ascension. The US had played only a supporting role in the First World War and had subsequently refused to join the League of Nations. As the fascist threat grew, the isolationist America First movement advanced – but the Japanese attack on Pearl Harbor on 7 December 1941, provoking the US’s belated entry into the Second World War, ended any ambiguity over its status.

At the end of the war, with so much of Europe devastated, the US acted as the guarantor of a new international order, exemplified by the Marshall Plan, the United Nations, Nato and the Bretton Woods institutions. Though the world’s superpower did not always live up to its high ideals, with catastrophic interventions in Vietnam, Latin America and the Middle East, it underwrote a rules-based, liberal global order. The US’s hallowed constitution and democratic principles also served as an inspiration to other nations.

Under President Donald Trump, the US has relinquished this status. As the historian and author of the acclaimed American Caesars, Nigel Hamilton, writes in this week's magazine (to be published online on 4 September): “The American empire… was created by a single American commander-in-chief [Roosevelt] – arguably the greatest leader in American history – and is now being systematically dismantled by another American commander-in-chief.”

Far from promoting and defending democracy abroad, Mr Trump is threatening it at home. His refusal to condemn the white supremacists who marched in Charlottesville, Virginia, drawing a moral equivalence between them and their anti-fascist opponents, betrayed his far-right prejudices and ignorance. He insulted the memory of those Americans who fought to defeat the Confederacy in the 19th century and fascism in the 20th century.

Rather than honouring his country’s democratic tradition, Mr Trump has empowered the dark forces of “the indigenous American berserk”, to use a phrase of the novelist Philip Roth. He has been constrained by Congress and the Supreme Court (thanks to the foresight of the US’s founding fathers) and has been subjected to relentless scrutiny by a free and sceptical media, but Mr Trump has used the so-called bully pulpit of the presidency for malign ends. Asked about the threat of impeachment, his close adviser Roger Stone warned: “You will have a spasm of violence in this country, an insurrection like you’ve never seen.”

As the president shreds his country’s moral legitimacy and emboldens white nationalists as no modern American leader did, other countries are ruthlessly taking advantage. A revanchist Russia tramples on rights at home and abroad. North Korea, undeterred by Mr Trump’s warnings of “fire and fury”, fired a ballistic missile over Japan on 29 August – its most provocative act in the Pacific region in recent years.

These are uneasy times. As Lawrence Freedman, one of the world’s foremost military historians, writes on page 34, Mr Trump and Kim Jong-un are “competing in bombast” and “capable of a rhetorical escalation that could turn into something truly unpleasant”. Nuclear stand-offs require perpetual patience, resourcefulness and tact – all qualities in which Mr Trump is distinctly lacking.

As the federal investigation into Russian interference during the 2016 US election continues, the president is ever more preoccupied with his own fate. By withdrawing from international agreements, such as the Paris climate accord, Mr Trump is performing what Nigel Hamilton calls an “Amerexit”. As a consequence, the stability of the postwar liberal world order is under threat.

Britain is similarly absorbed by the self-serving project of Brexit. Boris Johnson, the clownish Foreign Secretary, has failed to evolve a coherent foreign policy. And he shares with Mr Trump more than a ridiculous hairstyle: both men are narcissists and blowhards whose character flaws make them unfit to hold high office.

The world is left to contemplate the decline of Mr Roosevelt’s bequest and the spectre of a new era of great power rivalry in which “might makes right”. For all its moral and political defects, deep inequalities and rapacious pursuit of influence, the US has been the most benign superpower the world has known. Its successor may not be.