When Were we Great?

The Space Alien is curious about the use of the word "again" in the slogan of a current presidential candidate:  "Make America Great Again."  This slogan implies that America was only great at some earlier time.  So when, exactly, was America great in this speaker's opinion?

The Space Alien wonders if he means the Great Ice Age when glaciers covered the North American continent.  The Space Alien loves winter sports and truly enjoyed slipping and sliding over thousands of miles of frozen landscape in the company of friendly wooly mammoths during that chilly era.

Photographed by the Space Alien

Perhaps, however, he means the Mesozoic Era when dinosaurs roamed the Earth.  Some of the Space Alien's best friends were dinosaurs and they are sorely missed.

Photographed by the Space Alien

The Space Alien wishes politicians and others using this term would clarify as to exactly when in the past America was so great that we would need to return to that time.