This document summarizes trends in armed conflict, including the shift from interstate to intrastate wars that now mostly occur in the global south. Civil wars are more common and have causes like ethnic or religious conflicts, weak states, and large youth populations. Failing states threaten instability and their civil wars may become internationalized. Insurgencies aim to raise conflict costs rather than battlefield victories. Terrorism uses violence for political goals, and new terrorism is decentralized, lethal, and seeks weapons of mass destruction. Theories for conflict analyze factors from human nature to the international system's anarchy.