More than 3 million of these immigrants arrived from Ireland and Germany. Many of them were fleeing economic or political troubles in their native countries. Most immigrants arriving between the years of 1840 and 1860 were Irish. At that time, a disease called potato blight caused potatoes to rot.

The mid-19th-century wave of Irish and German immigration was driven by a combination of catastrophic environmental, economic, and political "push" factors. For the Irish, the primary driver was the Great Famine (1845–1852), caused by a potato blight that destroyed the staple crop of the tenant farmer population. Facing starvation and an indifferent British colonial government, over a million Irish fled to save their lives. In contrast, German immigrants were often "pushed" by a mix of political instability and economic displacement. The failed Revolutions of 1848 forced many political dissidents and intellectuals (known as "Forty-Eighters") to seek asylum in a democratic U.S. Additionally, the Industrial Revolution in Europe was destroying traditional artisanal trades, while a string of crop failures and land inheritance laws made farming increasingly unsustainable for the German peasantry. While the Irish primarily sought survival and basic labor, many Germans arrived with enough capital to establish farms or businesses in the Midwest, but both groups were united by the dire need to escape a rigid European social order that offered no path to prosperity or safety.