International humanitarian law does not say “try to avoid civilians if convenient.”
It sets out duties that bind every party, at all times, regardless of what the other
side does:
Distinction. You must at all times distinguish between combatants
and civilians, and between military objectives and civilian objects. Attacks that
are not directed at a specific military objective are indiscriminate, and
indiscriminate attacks are prohibited.
Proportionality. An attack expected to cause civilian death or
injury excessive in relation to the concrete and direct military advantage
anticipated is prohibited — even if the target itself is legitimate.
Precaution. Constant care must be taken to spare the civilian
population. That includes choosing means and methods of attack to minimise
civilian harm, and calling an attack off when it becomes apparent the harm would
be disproportionate.
Special protection for children. The Fourth Geneva Convention and
Additional Protocol I give children special respect and protection. The Convention
on the Rights of the Child — ratified by almost every state on earth — obliges
states to respect the rules of humanitarian law relevant to children and to
protect children affected by armed conflict. The UN Security Council has named
six grave violations against children in war, among them killing and maiming,
attacks on schools and hospitals, and denial of humanitarian access.
None of this is exotic or new. It is the minimum that states themselves wrote
down, signed, and told their own soldiers to obey.