International security has traditionally centered on state security and the perennial problem of war and similar forms of violent conflict. War has ended the lives of hundreds of millions of people and left once thriving societies in ruins. How and why international organizations, specifically IGOs, respond to war and threats to the peace is the focus of this chapter. The sources of conflict are numerous and intertwined. States fight for many kinds of reasons: territory, resources, regime change, ideology, nationalism, and religion. Yet the solutions to war are few and frustratingly elusive.