The end of the Cold War was supposed to deliver a long peace. For a while, that was working, until it wasn’t. We might now be entering another period when the breakdown of a world order leads to war. It was no accident that President Xi Jinping cited the “Thucydides Trap” during his summit in Beijing with his US counterpart Donald Trump, putting it forward as something for both superpowers to avoid. The theory, popularised by Harvard political scientist Graham Allison, digs deep into an episode...