With Congress again failing to curb US President Donald Trump’s authority over Cuba, the centre of gravity in US policy towards the island is shifting decisively back to the White House – and to a narrower, more consequential question: what comes next?Tuesday’s 51-47 Senate vote to block a Democratic-led war powers resolution on Cuba does not change policy on its own. It was widely expected, reflecting a long-standing pattern in which Congress has struggled to assert its war powers authority...