The category of "nationals of the United States" was first defined under the Immigration and Nationality Act of 1940. However, long before the Act, the category of the "national" had become recognized as a necessary legal classification to deal with inhabitants of Puerto Rico, one of the newly acquired islands from Spain, as a result of War of 1898. The U.S. Congress provided Puerto Rico with a local civil government and treated the inhabitants of the island as "citizens of Puerto Rico" under the Foraker Act. Neither foreigners nor U.S. citizens, the citizens of Puerto Rico had remained "nationals" until passage of the Jones Act of 1917, where they were conferred U.S. citizenship.
The notion of the "national" has been thought to be something to symbolize unequal treatment toward the Puerto Ricans and other islanders. As Cabranes points out, "the status of national, as distinguished from citizen, became a convenient construct for those who favored territorial expansion but did not wish to make the people of the new territory citizens of the U.S. or otherwise suggest that they might aspire to equality under the American constitutional system." (Cabranes 6).
However, the legal notion of the "national" came to be in use not just to give a terminology to the degraded status of the Puerto Ricans, but as a necessary construct for McKinley's Republican administration to settle legal contradictions, which were arising from a question of sugar trade. Being afraid that if the U.S. decided to make the sugar from Puerto Rico free of duty, it would be trapped into importing duty-free sugar also from the Philippines, and ultimately Cuba, a big sugar exporter, the Republicans passed the Foraker Act to put a tariff on Puerto Rican sugar. The Democrats who defended free trade, especially those from the southern states, strongly criticized the Republicans' sugar policy, insisting that putting a duty made Puerto Rico a foreign island in spite of the fact that the island had ceased to be such, as a result of the cession in 1898. Therefore, the "national", an in-between status of foreigners and U.S. citizens, was necessary for the U.S. to settle the political and economical struggle between the Republicans and the Democrats over sugar.
Although having been U.S. citizens since 1917, the Puerto Ricans still have a distinct group identity because of their historical experience as nationals. Understanding the "national" helps us to understand the construction of multi-cultural aspects of the U.S. society.