From Prof David Potter.

Sir, While I heartily agree with David Walker that Roman history can offer profound insight into the reasons why a solitary superpower can fail ("America risks the fate of the Roman republic", August 22), I fear that he may not have known where to look for those insights. Moreover, since he offered no points of actual comparison to support his statement that "policymakers must learn certain lessons from history, notably the downfall of the Roman republic", readers may reasonably wonder what exactly he was thinking about.