This graph shows the number of immigrants that entered the United States between 1915 and 1950.
Number of Immigrants Entering the United States.
500,000
450,000
400,000
350,000
300,000
250,000
200,000
150,000
100,000
50,000
1915
1920
1925
1930
1930
1935
1935
1934
1945
1950
What accounts for the trend in this graph?