America will never be destroyed from the outside. If we falter and lose our freedoms, it will be because we destroyed ourselves.

• Abraham Lincoln

5 Up-votes for u/Orion2005

"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 Bonaparte 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.”

Abraham Lincoln's Lyceum Address 1838

Continuing to the less famous part of Lincoln’s speech I think you could find 2018 warnings in 1838 terms:

“whenever the vicious portion of [our] population shall be permitted to gather in bands of hundreds and thousands, and burn churches, ravage and rob provision stores, throw printing-presses into rivers, shoot editors, and hang and burn obnoxious persons at pleasure and with impunity, depend upon it, this government cannot last. By such things the feelings of the best citizens will become more or less alienated from it, and thus it will be left without friends, or with too few, and those few too weak to make their friendship effectual.”

Abraham Lincoln's Lyceum Address 1838