A combination of unemployment, famine and religious persecution drove more than 30 million European immigrants to the United States in the 19th and early 20th centuries. The first major wave of immigration to the U.S. occurred between 1820 and 1870, when a famine in Ireland and North Europe and economic troubles in Germany brought more than 7 million immigrants to America. But the height of immigration was between 1880 and 1920, when more than 20 million immigrants from Central, Eastern and Southern Europe, including millions of Russian Jews fleeing religious persecution, came to the U.S.. The open door immigration policy ended with the passage of the Emergency Quota Act in 1921 which set quotas according to nationality.

Severe Famine

As a result of a devastating famine, about half a million Irish immigrated to the U.S. to escape starvation and disease between 1845 and 1850, with millions more following suit by the end of the century. Social policies under British rule of Ireland, which prohibited Irish Catholics from entering professions or purchasing land, forced a large portion of the Irish population to rent small plots from British landlords. The poor Irish farmers largely subsisted on potatoes, which grew easily and abundantly in that region. After a fast-moving fungus began destroying the crop in the early 1840s, about half of Ireland was without its staple food, causing mass starvation. The British government -- arguing the free market would end the famine -- did little to aid the starving Irish population, something that would increase the bitter rift between the Irish and British for decades to come.

Economic Depression

During the mid-1800s more than one million Germans fled their homeland to escape poverty and political unrest. An economic depression halted industrial expansion in German cities, leading to plummeting urban employment. Famine drove lower class Germans to stage hunger riots and periodic violent disturbances until Germany was on the precipice of open rebellion. Those with the means to travel began to set their sights on America, albeit out of pure necessity – during the mid-19 century the U.S. was one of the only nations accepting German immigrants.

Religious Persecution

About 2 million Jews from Central and Eastern Europe arrived in the U.S. between 1880 and 1920. Although thousands of Central European Jews had emigrated to America during the first half of the 19th century, those numbers skyrocketed after Russia erupted into deadly anti-Jewish riots in response to the assassination of Tsar Alexander II. Rumors circulating during that period blamed Jews for orchestrating the tsar’s murder, eventually leading the government to adopt a policy of systematic discrimination against the country’s Jewish population.

Natural Disasters

The U.S. experienced a surge in Italian immigrants following a string of natural disasters that swept through southern region of the country in the early 1900s. The eruption of Mount Vesuvius in 1900, as well as an earthquake and tidal wave that decimated the waterway between Sicily and the Italian mainland in 1908 killed more than 100,000 people in the city of Messina alone. The fledgling Italian government – the entire peninsula was only unified as one country in 1861 – did little to aid peasants in the rural south and Sicily, a region that lacked key resources necessary for industrial and economic development. The government did not bring necessary aid to the south following the natural disasters of the early 1900s, instigating a wave of immigration to the Americas as transatlantic travel became more affordable.