In 1899 Rudyard Kipling, the pre-eminent poet of British imperialism, addressed some stanzas to America. “Take up the white man’s burden,” he urged – “The savage wars of peace/ Fill full the mouth of famine/ And bid the sickness cease.” These days America has a black president and no public intellectual would dare to use the imperialist language of a Kipling. But the idea that the US bears a special burden in policing the world is very much alive. The notion was there in Barack Obama’s call for military action over Syria: “We are the United States,” declared the president – outlining his nation’s special role in creating and defending the post-1945 global order.

But is America still prepared to play the role of world policeman and to wage the “savage wars of peace”? That question will hang over Congress’s debate on intervention in Syria. Mr Obama’s own hesitancy and opinion polls in the US underline that many Americans have grave doubts. They are likely to be reinforced by Britain’s decision to stay out of any military intervention in Syria. Almost 80 years after Kipling’s death, many in the UK have interpreted parliament’s decision as a signal that Britain has finally sloughed off the post-imperial instinct to police the world – even as deputy sheriff to the US.

Since the UK is the world’s fourth- largest military power, and a member of the UN Security Council, such a decision would have global ramifications. But if America were to take a similar path, it would be truly world-shaking. And yet the possibility is clearly there. The US is war-weary after Iraq and Afghanistan and its economy has been weakened by recession. The shale gas revolution has made the country much less reliant on the Middle East. Americans, from Mr Obama down, no longer harbour the illusion that their troops will be greeted with flowers in foreign countries. Instead, as Kipling warned, they have learnt to expect “the blame of those ye better/ The hate of those ye guard.”