If there is one thing Americans should have learnt from their recent wars, it is that they do not have the wisdom, resources or staying power to dictate political outcomes. Not long ago Washington aspired to build prosperous democracies in Afghanistan and Iraq. Today it would be satisfied if they simply hung together as countries. President Barack Obama says the US should recognise that the world is “messy”. His strategy has been to avoid doing “stupid stuff”.

And yet he is again trying to put a more ambitious face on American policy, asserting this month that the US would “degrade and ultimately destroy” the Islamic State of Iraq and the Levant, the al-Qaeda offshoot known as Isis. Air strikes quickly followed.

Yet he is overpromising. The US needs a more feasible strategy. Mr Obama could learn from England’s policy – and later Britain’s – towards the European continent over the centuries. London had no permanent friends. But whenever a single power looked set to dominate Europe, the country would throw its weight behind an opposing coalition, a strategy known as “offshore balancing”. Britain was never a land power but its navy and economic might turned the balance against would-be hegemons.