Unlike most European and Asian nations, the population of the United States is not of one specific ethnicity. Rather, people emigrated to America over the course of its existence and added their own traditions and diversity to the culture. During the 19th century, approximately five million people came to the U.S. for reasons as diverse as famine, political upheaval and the promise of riches or opportunity.

Irish Potato Famine

By the 19th century, the potato was the staple crop in Ireland because of its rapid growth and high calories per pound. In 1845, a Mexcian fungus infested the potato crop, causing all infected potatoes to rot within hours of harvest. As the Irish had come to depend on the potatoes for their main source of sustenance, this blight led to mass hunger on the island. Over the next five years, one-quarter of the Irish population left the country, with approximately 1.5 million coming to America.

Upheaval in Germany

Today, we think of Germany as a single country -- or at worst a pair of countries recently reunited. But the region was a decentralized collection of autonomous states until the late 1800s. These different areas shared a common culture and language, but were not a single nation until Bismark united Germany in a series of wars. These wars, and Bismark's hostility toward Catholic Germans, led 1.4 million Germans to flee the area and come to the United States during the 1880s and 1890s.

Industrial Revolution

During the late 19th century, Europe underwent a shift from rural living to urban living as a result of the Industrial Revolution. The creation of a factory work force allowed industry to produce goods faster than individual artisans, putting the artisans out of work. Opportunity to work in a smaller town that had not yet industrialized, or among the job options in growing American urban centers, led many European artisans to come to the United States during this period.

California Gold Rush

At the beginning of 1848, the territory of California was neither well-settled nor well-known. When gold was discovered that January, word spread quickly and millions of German, Australian, Chinese and Latin Americans traveled to California to seek their fortunes in 1849. While some of these "forty-niners" returned home after striking it rich -- or after failing miserably -- many stayed and ultimately became Americans.