I doubt we started this rhetoric clear back at the start. Was this like the whole under God thing? Were we trying to pump ourselves up during the cold war?
May not be the beginning, but in Abraham Lincoln's Lyceum Address (1838) he says the following:
"This task of gratitude to our fathers, justice to ourselves, duty to posterity, and love for our species in general, all imperatively require us faithfully to perform.
How then shall we perform it?--At what point shall we expect the approach of danger? By what means shall we fortify against it?-- Shall we expect some transatlantic military giant, to step the Ocean, and crush us at a blow? Never!--All the armies of Europe, Asia and Africa combined, with all the treasure of the earth (our own excepted) in their military chest; with a Buonaparte for a commander, could not by force, take a drink from the Ohio, or make a track on the Blue Ridge, in a trial of a thousand years.
At what point then is the approach of danger to be expected? I answer, if it ever reach us, it must spring up amongst us. It cannot come from abroad. If destruction be our lot, we must ourselves be its author and finisher. As a nation of freemen, we must live through all time, or die by suicide."
https://www.abrahamlincolnonline.org/lincoln/speeches/lyceum.htm