In order to present a hopefully comprehensive framework of what is the stake of the growing use of robot soldiers, the paper focuses on: (1) the different impact of robots on legal systems, e.g., contractual obligations and tort liability; (2) how robots affect crucial notions as causality, predictability and human culpability in criminal law and, finally, (3) specific hypotheses of robots employed in "just wars." By using the traditional distinction between causes that make wars just (i.e., bellum iustum) and conduct admissible on the battlefield (i.e., ius belli), the aim is to clarify how advancement of military robotics technology is transforming a 2,000-year-old legal debate on the concept of "just war." For the first time, legal systems will hold political authorities and military commissioners responsible for what an artificial soldier autonomously decides to do. The paper examines how the new scenario affects both principles of military conduct and notions of justice in resorting to war.