Final answer:
The region that experienced a decline in immigration to the United States after 1919 is Europe, as confirmed by historical trends and restrictive immigration policies that targeted European immigrants, especially from southern and eastern Europe.
Step-by-step explanation:
Between 1820 and 2010, the region that experienced a decline in immigration to the United States after 1919 is Europe. This was a period when the United States saw a significant decrease in European immigrants, especially following the turbulent times of World War I. Initially, European immigrants came from western and northern Europe, but by the end of the 19th century and beginning of the 20th century, there was a large influx from southern and Eastern Europe. These newcomers often sought to escape poverty, violence, and natural disasters, and many were drawn to the promise of employment and land in the U.S. However, the number of immigrants from Europe started to decline from the 1920s, partly due to restrictive immigration policies and quotas directed particularly towards southern and eastern Europeans. The U.S. Department of Homeland Security's Yearbook of Immigration Statistics confirms this trend with a decline in European immigration starting from the early 20th century through the following decades.