 When wars happen, international humanitarian law sets out agreed rules for how they are to be conducted. States develop these rules to protect the civilian population from the effects of hostilities. These rules outline what targets may be legally attacked and how, based on a careful balance between military necessity and humanity. Like all of IHL, they apply regardless of the reasons for fighting. The three main principles are distinction, proportionality and precautions. The principle of distinction requires parties to armed conflicts to distinguish at all times between the civilian population and combatants and between civilian objects and military objectives. Attacks against military objectives including combatants are not prohibited. In contrast, the civilian population, individual civilians and civilian objects must never be attacked. Direct attacks against them are prohibited, as are indiscriminate attacks, that is, attacks that by their nature strike military objectives and civilians or civilian objects without distinction. Under the principle of proportionality, attacks against military objectives are prohibited if such attacks may be expected to cause incidental loss of civilian life, injury to civilians, damage to civilian objects or a combination of these, which would be excessive in relation to the concrete and direct military advantage anticipated. In simple terms, this means that a military objective may only be attacked if the potential accompanying civilian losses are not expected to outweigh the foreseen military advantage. The principle of precautions requires that in the conduct of military operations, constant care must be taken to spare the civilian population, civilians and civilian objects. In attack, so-called active precautions include doing everything feasible to verify that targets are indeed military objectives, cancelling or suspending an attack if it becomes apparent that the target is not a military objective and giving effective advanced warning of attacks which may affect the civilian population unless circumstances do not permit. So-called passive precautions require taking all feasible steps to protect the civilian population and civilian objects under one's control against the effects of attacks. The principles of distinction, proportionality and precautions are complementary and all three must be respected for an attack to be lawful. Distinction, proportionality and precautions, like IHL in general, do not stop wars from happening but these three principles work together as a tool to limit suffering and destruction. By outlining rules for the conduct of hostilities, it is possible for international humanitarian law to prevent or at least reduce war's most devastating effects.