The huge changes that occurred in the United States over this time period were more economic and social than they were political. However, there were the beginnings of important political changes by 1900.
The time after the Civil War is generally described as a time when America was becoming much more industrialized. This industrialization was driven by the expansion of the railroads. As the railroads expanded, they caused an increased demand for steel. They also created a national market because they allowed people in all parts of the US to trade with one another. This allowed businesses to increase in size. This increase was typified by such huge firms as Standard Oil, owned by John D. Rockefeller.
A great deal of social change came along with this economic change. America became much more urbanized. There came to be a large working class living in the cities. Many of these people, particularly at the end of the 1800s, were immigrants. This was a huge change from the days when the US was a more rural nation.
The political changes were not as dramatic. However, by 1900, there was the start of what is now called the Progressive Era. The economic and social changes had caused middle class Americans to want to push back and demand more control over their government. This led to more regulation of the economy and of society. However, this change was not very advanced yet in 1900.