T he US is a great nation, possibly the greatest of all time. Yet to keep America great, policymakers must learn certain lessons from history, notably the downfall of the Roman republic.

The world has changed dramatically in recent years. The US is currently the sole superpower on earth but that exclusive status is likely to be short-lived. While the US is number one in many things, from the size of its economy to military might, it faces several big sustainability challenges.