This article develops the concept of universal harm as a framework for understanding the transformative impact of contemporary warfare on human, ecological, and biospheric conditions of life. Moving beyond traditional legal approaches grounded in necessity, proportionality, and distinction, the study argues that modern forms of armed conflict – particularly those involving nuclear weapons and depleted uranium – generate forms of damage that transcend individual victims, national borders, and generational limits. Through an analysis of key judicial developments, including Japanese jurisprudence on the Hibakusha, the Advisory Opinion of the International Court of Justice on nuclear weapons, and recent reflections on climate change and erga omnes obligations, the article highlights the persistent difficulty of international law in recognizing and addressing the universal scope of such harm. Particular attention is devoted to the precautionary principle and to the challenges posed by scientific uncertainty and causal attribution in cases involving environmental and radiological damage.