Q: What is a lawful military objective?
A: The determination of what a lawful "military objective" is turns on an assessment of "military advantage." It is defined as "objects which… make an effective contribution to military action and whose partial or total destruction, capture or neutralization…offers a definite military advantage." Sites that may once have been purely civilian can be turned into legitimate military objectives. The perspective is that of the commander in the field at the time of a targeting decision, with the information then available.
Military manuals also recognize that objects "normally dedicated to civilian purposes, but which are being used for military purposes" (such as houses, schools or churches) lose their protection under the applicable law, and may properly become lawful "military objectives." Attacks must not be "indiscriminate," that is, untargeted, launched without consideration as to where harm will likely fall.
A commander's intent is critical in the principle of distinction during armed conflict. Where it is believed in good faith, on the basis of the best available intelligence, that a civilian building has been misused as a sanctuary for military fighters, military intelligence, or the storage and manufacture of military assets, the commander has a legitimate basis for using force against the site.
Q: What about the accusation of war crimes?
A: Mistakes made in armed conflict do not, as such, constitute war crimes. As two leading scholars have recognized, "[t]he prerequisite for a grave breach is intent; the attack must be intentionally directed at the civilian population or individual civilians, and the intent must embrace physical consequences." Military operations that cause unintended and unwanted damage to civilians do not constitute violations of the Law of Armed Conflict, much less a war crime.
While Hamas deliberately sought to harm civilians by launching rockets and mortars on towns in southern Israel, and even boasted about directing their attacks at civilian populations, the IDF carefully checked and cross-checked targets to make sure they were being used for combat or terrorist activities, and not solely for civilian use.