内战后美国政府鼓励民众向西移民，还鼓励修筑铁路，连接西部至经济较发达的东岸地区。同时西部不少地方自1850年代都发现了金矿，吸引不少居于东岸的人淘金。1869年连接美国东西海岸的铁路竣工，铁路带给美国原居民更多冲突，当当地土著首领和移民签订土地协议时，双方对协议内容有不同理解。土著不能理解移民的私有财产理念，他们认为自己能在土地上打猎。一场为生存而战的战斗开发了，许多土著买了枪械，袭击移民宅地、马车队、铁路和美国骑兵。而美国白人则大量屠杀土著赖以为生的野牛，由1860年的1500万头，到1885年只剩下2000头。土著被迫在贫瘠的土地上种植庄稼，他们不习惯种植，不能成为美国公民。土著与美军长期战斗，使土著几乎灭亡。1890年，最后一次战斗屠杀了200多名苏族人。不久后所有的土著都被赶进保留地。土著美洲人的生活方式永远结束。
这段时间美国利用其充足的人口、国内工业发展和一系列的国际投机活动，令美国国际影响力迅速攀升。在十九世纪末期，美国已成为世界工业大国，并出现新的科技（如电报和贝塞麦(转炉)炼钢法），兴建更大的铁路网络。
在这个时期，美国也成为国际性参与者。在1900-1903征服了超过一百万的菲律宾人，并屠杀了大量菲律宾人。
Reconstruction took place for most of the decade following the Civil War. During this era, the "Reconstruction Amendments" were passed to expand civil rights for black Americans. Those amendments included the Thirteenth Amendment, which outlawed slavery, the Fourteenth Amendment that guaranteed citizenship for all people born or naturalized within U.S. territory, and the Fifteenth Amendment that granted the vote for all men regardless of race. While the Civil Rights Act of 1875 forbade discrimination in the service of public facilities, the Black Codes denied blacks certain privileges readily available to whites. In response to Reconstruction, the Ku Klux Klan (KKK) emerged around the late 1860s as a white-supremacist organization opposed to black civil rights. Increasing hate-motivated violence from groups like the Klan influenced both the Ku Klux Klan Act of 1870 that classified the KKK as a terrorist group and an 1883 Supreme Court decision nullifying the Civil Rights Act of 1875; however, in the Supreme Court case United States v. Cruikshank the Court interpreted the Fourteenth Amendment as regulating only states' decisions regarding civil rights. The case defeated any protection of blacks from terrorist attacks, as did the later case United States v. Harris. During the era, many regions of the southern U.S. were military-governed and often corrupt; Reconstruction ended after the disputed 1876 election between Republican candidate Rutherford B. Hayes and Democratic candidate Samuel J. Tilden. Hayes won the election, and the South soon re-entered the national political scene.
Following was the Gilded Age, a term that author Mark Twain used to describe the period of the late nineteenth century when there had been a dramatic expansion of American industry. Reform of the Age included the Civil Service Act, which mandated a competitive examination for applicants for government jobs. Other important legislation included the Interstate Commerce Act, which ended railroads' discrimination against small shippers, and the Sherman Antitrust Act, which outlawed monopolies in business. Twain believed that this age was corrupted by such elements as land speculators, scandalous politics, and unethical business practices. By century's end, American industrial production and per capita income exceeded those of all other world nations and ranked only behind Great Britain. In response to heavy debts and decreasing farm prices, farmers joined the Populist Party. Later, an unprecedented wave of immigration served both to provide the labor for American industry and create diverse communities in previously undeveloped areas. From 1880 to 1914, peak years of immigration, more than 22 million people migrated to the United States. Abusive industrial practices led to the often violent rise of the labor movement in the United States. Influential figures of the period included John D. Rockefeller and Andrew Carnegie.