"A people gather, grow strong under adversity, weaken under prosperity, and fall, first victims of weakness within and then victims of strength without." 

-Calvin Coolidge

Why does prosperity weakens societies? 

History is clear that this is real, but why? 

The answer is instructive: Society is strong when the people and leaders are diligent in maintaining the things which made the society strong. 

Under prosperity, at first, the people feel that their successes can be lost and so they remain diligent.

Over time, however, the old generation that achieved prosperity out of adversity dies off, and their children and grandchildren benefit from peace and prosperity without having to personally pay for its achievement.

These later generations come to feel entitled to prosperity, without understanding what it cost in the first place. 

The diligence of the people wanes, and apathy sets in. 

During all this, the trend of prosperity continues and increases. The gap between prosperity and character widens.

But Coolidge concludes with this thought: 

"Nor need it unduly alarm us." 

He knows the times of American weakness are coming, but he is not alarmed because such weakness brings adversity and a chance for a new generation of Americans to rise to the occasion.