In 1938, at a dinner party, the American ambassador in London, Joseph Kennedy, an appeaser through and through, predicted to Churchill that England would go under. It drew from Churchill an impromptu oration that included these words:
It will then be for you, for the Americans, to preserve and maintain the great heritage of the English-speaking peoples. It will be for you to think imperially, which means to think always of something higher and more vast than one's own national interests.*

To think always of something higher and more vast than one's own national interests... It does describe how to think imperially; but it also describes any well-thought out national policy.

A light unto the nations.

Who observes that light is produced with heat - the physicist? The cynic?

* Sorry, but I forgot the article I copied this from.