The Mariana Islands (also Marianas Islands) are an archipelago in the north-west Pacific Ocean, divided between the Commonwealth of the Northern Mariana Islands and Guam, both unincorporated territories of the United States.
World War II
In World War Two in the Pacific, the Mariana Islands were treated as one group. They cover a significant area, mostly water, of Micronesia in the Central Pacific. There were staged campaigns against Japanese bases in the more northern and more southern areas, but divisions between north and south have changed in today's political context.
The militarily significant north, in 1943 and early 1944, the targets of Operation FLINTLOCK, were Maloelap, and Wotje in the Ratak chain, and in the Ralik chain, Jaluit, Kwajalein Atoll, and Eniwetok Atoll. Jaluit was a seaplane base, all of the other Ratak sites were airfields, and the Ralik locations were anchorages for naval ships.  Subsequently, in mid-1944, Operation FORAGER was directed at Guam, Saipan, and Tinian.
Today, however, the Commonwealth of the Northern Mariana Islands contains Saipan and Tinian, but Guam has a separate government. Is Guam, then, the Southern Marianas? There are, perhaps, cultural reasons for saying that is the case, but the situation also has the aspect that any unification might mean that some with political power might need to relinquish it. 
Guam is about 50 miles south of the island of Rota, about 110 miles south of the island of Tinian and about 128 miles south of the island of Saipan.
I believe that if you ask any Chamorro living on the island of Guam if we consider ourselves the people of the Southern Mariana Island of Guam you may get a cold stare right back. In fact you will probably get a lot of resistance from the Guam people if you refer to them as Micronesians. To Guam people, Micronesians are those from the islands south or southeast of Guam.
- , PART III: The Marshalls: Quickening the Pace; Chapter 1 FLINTLOCK Plans and Preparations GETTING ON WITH THE WAR, History of U.S. Marine Corps Operations in the Second World War, U.S. Marine Corps Historical Center Branch, pp. 117-119
- Felix Aguon (16 May 2006), Why Guam and the Northern Marianas are Divided, Pacific Islands Development Center/East-West Center, associated with the University of Hawaii