We should note first of all that the Pacific Realm is rather a large place and different areas of that realm have different historical geographies. However, we can say some things about what is common to most of the realm.
The realm was populated, in human terms, by people coming from the mainland of Asia. There are typically said to have been three waves of migration that resulted in three somewhat different types of people in the Pacific Realm. These different peoples are Melanesians, Polynesians, and Micronesians. For the second two types, in particular, historical geography was determined by long ocean voyages. These people sailed long distances in canoes, populating new islands as they went.
The next big event in the historical geography of this realm was the coming of Europeans. As Europeans explored the Pacific, they claimed, and in some cases occupied, the various islands. Even on islands that were not occupied, the coming of Europeans changed native ways of life forever. Many of the islands of this realm were then dominated by Japan for all or part of the period between the world wars.
Since the end of WWII, the historical geography of the realm has generally included independence for most of the parts of the Pacific Realm that are (unlike New Zealand, for instance) still largely occupied by their original inhabitants.