Final answer:
There were no limits on European immigration to the US before WWI due to the demand for labor from industrialization. Restrictions were introduced in the 1920s due to changing attitudes and the desire to preserve American homogeneity. Western Hemisphere immigration remained unrestricted due to political and economic considerations.
Step-by-step explanation:
Immigration Before WWI and the Introduction of Quotas
Before World War I, there were no limits on European immigration to the US because the economic expansion due to industrialization created a demand for labor, which immigrants from Europe helped to meet. As America transitioned into a global industrial power, there arose a need for this labor force in the manufacturing belt's factories. This need for labor, along with America's traditional view as a land of opportunity, facilitated the influx of immigrants, who were also fleeing from various push factors in Europe such as poverty, war, and oppression.
It was only after the war that attitudes began to shift. The Emergency Immigration Act of 1921 and the National Origins Act of 1924 introduced numerical limits for the first time, with quotas based on past census data favoring immigrants from northern and western European countries. This shift toward immigration restriction was fueled by a desire to preserve American homogeneity and by the rising tide of nativism and xenophobia, especially toward immigrants from Southern and Eastern Europe.
The separation between Western Hemisphere immigration policy and that for the rest of the world is attributed to political compromises, with the former remaining unrestricted due to America's commitment under the Monroe Doctrine and industry demand in the Southwest. Isolationism also played a role in influencing immigration policy during this period.