I don't know that the country is not fragile, so much as the turmoil that could lead to real, violent conflict simply hasn't happened yet. A very big difference from pre-1860 America, or even Vietnam War-era America, is that there hasn't emerged an enormous cause to prompt the citizenry to start shooting and bombing each other.
But, my sense is that all the technological progress, and our much greater ability to control our environment and our own circumstances, hasn't done that much to override our capacity for emotional and mental mishaps. I don't think there's a truly rational explanation for why North Koreans and South Koreans live in such different circumstances. In 2014, did anyone in Venezuela imagine they'd be in a famine within 3 years? Or, for what the left seems to consider a similar-sized disaster, did any liberal in 2014 think Trump had a chance of becoming president?