Civil airports, roads and bridges are civilian objects that become military objectives subject to attack if they are actually used for military purposes or military objectives are located on or within them. Even then, the rule of proportionality applies, requiring the parties to the conflict to weigh the short- and long-term harm on civilians against the military advantage served; they must consider all ways of minimizing the impact on civilians; and they should not undertake attacks if the expected civilian harm outweighs the definite military advantage.