Donald Trump is the first US president to visit China in almost ten years. He will arrive for his summit exasperated that he has achieved none of his many and varied war aims in Iran, and having been humiliated by a weaker but dogged regime that survives in Tehran. His increasingly drawn-out conflict with Iran is hammering his approval ratings at home and leaves the Republican desperate for a visible foreign policy win. World Affairs Editor Sam Kiley analyses how Trump will be in a position of weakness during his meeting with Xi Jinping.