Political and economic instability and devastation were mostly contained to the South. Even before the Civil War, most of the population and economic growth were in the northern states. Population was rapidly growing in part due to natural high birthrate but also because of immigration. So the total population of the US was higher in 1870 than in 1860, despite the war. (About 600k people died as a result of the war but that was only 2-3% of the total population - not enough to significantly affect the demographics of the country). By 1900, the population had more than doubled compared to pre-war levels.

In terms of the economy, again the northern states were far more important. Coal, steel, oil, railroads, textiles and other industries were the engines of growth during the Industrial Revolution. The US was actually growing faster than European power during this time, thanks to cheap labor, land, natural resources and low taxes. Thus by 1900, the US GDP was higher than that of any European country. It was second only to the British Empire but only if you count the entire empire.

America had vast resources and the impetus of immigration which brought millions of people who were ambitious to start their own business and have their children educated. The one thing America had was jobs and resources. The America that we know was about to begin. It wasn't long before there were cars for example. Manufacturing jobs grew exponentially as the electric age burst upon the world and the age of invention took off. Made in the USA stood for quality and innovation... The 'what will they think of next' generation encountered the beginnings of the modern world ... from railroads to radio!