Detention in non-international armed conflicts (NIACs, or wars fought between States and non-State armed groups) is a time-honored military and humanitarian necessity. And yet, the principles of sovereignty, the texts of the law of armed conflict and international human rights law and the historical record leave little doubt: international law recognizes no inherent detention power in such wars. As long as NIACs were purely internal civil wars, there was little basis to question the exclusive role of domestic law in regulating detention of the enemy. With the advent of transnational NIACs, such as the war in Afghanistan involving multi-national forces against the Taliban and Al Qaeda, the legal playing field becomes more complicated. State practice has lacked coherence. Still, domestic law, as tempered by due process requirements of international human rights law, remains the most rational source of regulation for NIAC detention.