In US Supreme Court case of Fleming v. Page (1850), it was determined that:

So long as Congress has not incorporated the territory into the United States, neither military occupation nor cession by treaty makes the conquered territory domestic territory,...but those laws concerning 'foreign countries' remain applicable to the conquered territory until changed by Congress.

For those territories over which Spain gave up her sovereignty as a result of the April 11, 1899, Spanish-American Peace Treaty (Treaty of Paris), the landmark ruling of Downes v. Bidwell (1901) introduced the concept of "unincorporated territory" into the United States legal lexicon.

In other words, the US Supreme Court determined that upon the termination of Spanish sovereignty over these territories, under US law they became "US unincorporated territories." However, at the time that the Treaty of Paris came into effect (and indeed for several years thereafter in most cases), all of these territories were under United States Military Government (USMG), and not under any form of "civil government."

Hence, beginning with the Spanish-American War cessions, what the US Supreme Court is speaking of is the category of "unincorporated territory under USMG." Clearly, the three fundamental criteria for the recognition of this most basic type of US insular area are -- conquest by US military forces, the US as "the (principal) occupying power," and territorial cession in the peace treaty. (The issue of whether there is a "recipient" for the territorial cession in the peace treaty is a separate consideration.)

Hence, the earliest recognition of US insular areas included four: Puerto Rico, the Philippines, Guam, and Cuba -- all of which were under United States Military Government. According to the historical record, civil government authorized by the US Congress was achieved on the following dates: Puerto Rico - May 1, 1900; Philippines - July 4, 1901; Guam - July 1, 1950; and Cuba - May 20, 1902.

Cuba became independent on May 20, 1902, but the other three territories continued as US unincorporated territories, each with a civil government in place as of the date indicated. Later the Philippines became independent on July 4, 1946.

CONCLUSION: Under US law, the earliest formulation of a "US insular area" as recognized by the US Supreme Court is the category of "unincorporated territory under USMG."


More information:
    Unincorporated Territory under USMG (pdf)