In the middle of the nineteenth century, more than one-half of the population of Ireland moved to the US. An equal number of Germans came because of civil unrest, unemployment and hardships at home.
After the Germans and Irish moved to the U.S., hostility towards them erupted. One of the main reasons was due to Religion, all the Irish and most Germans were Roman Catholic. Another reason was political; most immigrants became democrats because the party focused on the needs of commoners. Some opposition was due to the fact that Americans in low-paying jobs were threatened and often replaced by immigrants willing to work for almost nothing in order to survive.
Ethnic and anti-Catholic rioting occurred in many Northern cities, the largest occurring in Philadelphia in 1844 during a period of economic depression. Protestants, Catholics and local militia fought in the streets; people died, some were injured and buildings demolished. The Nativist political parties formed almost overnight. They wanted to prevent foreign born people from ever holding public office or to have the same rights as other citizens. The most influential of these parties, the Know Nothings, was anti-Catholic.
Source: Irish and German Immigration
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