The Spanish East Indies were a large territory comprising mainly the Philippines,1 plus a large scattering of Micronesian islands in four main groups (the Marianas, the Carolines, the Marshalls,2 and the Palaus). Following Spain’s utter defeat in the 1898 Spanish-American War, the United States annexed (at least temporarily) almost the entirety of the Spanish colonial empire, including parts of the Spanish East Indies (the Philippines,3 plus the island of Guam4 in the southern Marianas); however, apart from Guam, Spain’s Micronesian holdings were left untouched, becoming the country’s only surviving colonial possessions outside of Africa.5
Given that the rest of Spain’s possessions in the East and West Indies were annexed by the United States, either temporarily (Cuba) or (quasi-)permanently (Puerto Rico, the Philippines,6 Guam), and that the U.S. did make an inroad (albeit a small one) into Spanish Micronesia by annexing Guam, why didn’t the U.S. go the full hog and annex the rest of Spain’s Micronesian islands?
1: Over which, however, Spain had little to no effective control outside of Manila.
2: Which were disputed with Germany.
3: Conveniently ignoring the fact that the independent Filipino government, an ally of the U.S. during the war, controlled literally the entirety of the Philippines except for the city of Manila, and quickly sparking another war, with hostilities continuing into the early 1930s in parts of the islands.
4: Which is why Guam is a separate territory from the Northern Mariana Islands (and also the reason the latter territory has the “Northern” in its name, as Guam is also one of the Mariana Islands - the southernmost one).
5: Having become essentially ungovernable with the loss of the colonial machinery in the Philippines, they were sold to Germany the following year, captured by Japan at the beginning of World War I, and eventually seized by the U.S. during (the Marshalls, Marianas, and Palaus) / at the end of (the Carolines) World War II (along with Guam and the Philippines, which had been overrun by Japan by a few days and a few months, respectively, after the outbreak of hostilities between the two countries in December 1941); following the latter war, they were assigned to the United States as a United Nations trust territory, most of which became independent in stages in the 1980s and 1990s as the countries of the Marshall Islands, the Federated States of Micronesia, and Palau, although the Marianas voted to instead become a U.S. commonwealth.
6: Yes, I am aware that the Philippines were granted self-government in the 1930s and full independence following World War II; however, as the original object of the United States had been to permanently annex the Philippines, and it continued to do so for a quarter-century after they first landed troops there, I’m putting the Philippines into the “(quasi-)permanent” category.