On the world stage, Australia ranks as a middle power at best. But among the islands of the Pacific Ocean, Australia is almost a superpower — serving as a major source of aid, a hungry market and a stalwart defense ally for these tiny nations. This is especially true in the subregion of Melanesia, which has long been a key backstop for U.S. dominance in the Asia-Pacific. Australia and New Zealand each have a strong incentive to bolster Washington's power in the subregion both to amplify their own positions on the world stage and prevent security threats to their respective countries. But China's increasing presence in Melanesia's resource-rich economies and infrastructure outlays, along with signs of nascent military ambitions in the area, are now threatening Australia's and New Zealand's preeminence in the region — and, by extension, that of the United States.