Prior to the Civil War, immigration was surging in particular from Germany, Ireland, and some Nordic countries.
During the 1830's-1850's, the US experienced a surge of new immigrants coming to the US looking for work. The market revolution offered opportunities for unskilled, poor immigrants to get jobs. those with more money were able to take advantage of new lands opening in the west (now Mid-west) for cheap.
Irish immigrants tended to be poorer and would come to the urban areas to settle and look for jobs. They created neighborhoods suited to their culture. Nativist groups formed in reaction to the new immigrants in particular the Irish. They were seen as unclean and as low in status as free blacks. Germans and those from Nordic countries tended to have more money and were skilled in farming. These groups came for cheap land and would make up the populations of Wisconsin, Illinois, Iowa, Minnesota, Indiana, and Ohio. Those that could buy land were able to create ethnic communities and were not bothered by nativist groups as much as those settling in the cities. During the years of the war, immigration slowed to a halt and would revive again to a full roar after the war ended.
There were many actions and characteristics of Teddy Roosevelt that contributed to his reputation as the first modern president, but one of the biggest was that his set aside land for national parks.
The united states power was continuing to grow in the international arena. Previous decades had seen the USA emerge as one of the industrialized country which implied that it could not remain isolated as a result of the dynamics of international commerce. The USA had to protect its interests in the global arena especially trade.
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I know one which the Phoenician's passed on their culture.
