Puerto Rico was annexed by the United States in the aftermath of the Spanish-American War of 1898. Puerto Ricans are not, strictly speaking, immigrants. Few Puerto Ricans migrated to the United States after World War II. There was extreme poverty in Puerto Rico. The postwar developed cheap airline fairs between Puerto Rico and the United States, about 50 dollars, two weeks' wages. Cheap fares enabled 10's of thousands of Puerto Ricans to come annually.
http://www.history.com/this-day-in-history/us-takes-control-of-puerto-rico

Immigration Act

Between 1952 and 1960 about 45,000 Japanese immigrated to the United States. 40,000 of them (85.9 percent) were woman who married non-Japanese soldiers and former soldiers. After the early 1960's immigration slowed. Later in 1965 the Immigration Act was passed, opening the doors for many Asian ethnic groups. The Japanese desire to come to the states after the Act was passed was not high.
http://history.state.gov/milestones/1945-1952/ImmigrationAct

The Gold Rush of 1849

The Gold Rush of 1849 attracted many immigrants to California. Immigrants came from all over to get a part of this luxurious finding. There were immigrants from Mexico, Chile, and other parts of Latin America flooding California to get some gold. Even the immigrants that were scattered throughout the United States were leaving where they were at to come to California. About $81million was pulled from the ground within the first 3 years of mining, and would continue through the years. Even to this day it is said that there is still gold out there for the mining.

http://www.history.com/topics/gold-rush-of-1849

Ellis Island

Ellis Island was an "island of hope" for most immigrants and an "island of tears" for the minority of immigrants coming to the states between the late 1800's and early 1900's. For those who were not admitted or those being deported, Ellis Island served as a detention center and would later serve as an internment center for many enemy aliens. Ellis Island has a broad history, which stands as one of the bigger gateways to the United States of America, where more than 12 million immigrants came through.
http://www.ellisisland.org/genealogy/ellis_island_history.asp