According to an uncontroversial principle of customary international humanitarian law (IHL), parties to an armed conflict must distinguish between the civilian population and combatants and between civilian objects and military objectives. In order to spare civilians and the civilian population from hostilities and their effects, it is essential to define who and what may be attacked. The first rule regarding attacks (by acts of violence2 ) is that the intended target must be a military objective. Once a military objective is the target, under additional rules, which are not discussed here, the attack may nevertheless become illegal if excessive collateral damage affecting civilians or civilian objects must be expected. Furthermore, even when attacking a lawful target, precautionary measures to spare civilians have to be taken. While the main aim of the law is to protect persons, it is appropriate to discuss first what objects may be attacked. This permits to clarify the criteria, which make targets legitimate. In addition, attacks on objects involve the greatest danger for persons who are beyond any doubt civilians.