Oceania ( or ) is a geographic region comprising Melanesia, Micronesia, Polynesia and Australasia. Spanning the eastern and western hemispheres, Oceania covers an area of and has a population of 40 million. Oceania is the smallest continental grouping in land area and the second smallest in population after Antarctica. 

The islands at the geographic extremes of Oceania are Bonin Islands, a politically integral part of Japan; Hawaii, a state of the United States; Clipperton Island, a possession of France; the Juan Fernández Islands, belonging to Chile; the Campbell Islands, belonging to New Zealand; and the Cocos (Keeling) Islands, belonging to Australia. Oceania has a diverse mix of economies from the highly developed and globally competitive financial market of Australia and New Zealand, which rank high in quality of life and human development index, to the much less developed economies that belong to countries such as of Kiribati and Tuvalu. The largest and most populous country in Oceania is Australia, with Sydney being the largest city of both Oceania and Australia. 

The first settlers of Australia, New Guinea, and the large islands just to the east arrived between 50,000 and 30,000 years ago, when Neanderthals still roamed. Oceania was first explored by Europeans from the 16th century onward. Portuguese navigators, between 1512 and 1526, reached the Tanimbar Islands, some of the Caroline Islands and west Papua New Guinea. On his first voyage in the 18th century, James Cook, who later founded the Hawaiian Islands, went to Tahiti and followed the east coast of Australia for the first time. The Pacific front saw major action during the Second World War, mainly between the belligerents United States, its ally Australia, and Japan.
who were the belligerents?
 United States, its ally Australia, and Japan.