This study explains why Indonesia in cooperation with the Melanesian Spearhead Group. Members of the Melanesian Spearhead Group are the countries in the South Pacific region which has a Melanesian race. The vision of the Melanesian Spearhead Group is decolonization and freedom for all countries of Melanesia with the efforts to develop cultural identity and association, political, social and economic of Melanesians. United Liberation Movement of West Papua is listed as an observer in the Melanesian Spearhead Group. As an associate member, Indonesia tried to prevent the movement of groups of Papuan independence as part of the Melanesian Spearhead Group. Indonesia's involvement in the Melanesian Spearhead Group becomes a threat to the Papuan independence groups to secede from the Unitary Republic of Indonesia. Through the concept of foreign policy with Rational Actor model by Graham T. Allison, that there are alternatives to policy guidelines that could be taken by the government in the calculation of gains and losses over each of these alternatives. The decision-makers must always be ready to make changes or adjustments in its discretion. This concept is used by researchers to determine the involvement of Indonesia in the Melanesian Spearhead Group. The main reason why Indonesia in cooperation with the Melanesian Spearhead Group is to gain the support of the member countries of the Melanesian Spearhead Group to prevent group of Papuan independence became a permanent member to inhibit movement in liberating Papua.