Relations among States have for long been characterised by conflicts that have led to resort to use of armed force among them. However, history has also consistently seen efforts to regulate the use of armed force in hostilities. That process came to a climax in the twentieth century when war was declared a non-essential feature of statecraft. During the last century, recourse to use of armed force was practically brought under unprecedented pressure by international legal formulations as well as the general ethical regard for such war, which hitherto had been a symbol of valour, honour and Statehood. In the wake of this, just cause for interstate war took on a new meaning. This book provides an explanation and definition of that new turn. As such, it should help academicians, researchers and historians who ply into the reasoning behind armed conflicts among States, or anyone considering what could legally and ethically make for a just cause for interstate wars today.