Bougainville Island with Papua New Guinea, Australia, The Solomon Islands, and Vanuatu. You can see that Bougainville belongs with the Solomon Islands geographically (as the northernmost in the chain), but because of international land swapping after WWI it went to Australia and thus was included in Papua New Guinea when that country became independent. Bougainvilleans are culturally and biologically distinct from other Papua New Guineans, and it's easy to see why.