ICJ orders Israel to prevent genocide in Gaza, stops short of ordering ceasefire

International law regulates war, but does not entirely prohibit it (that would be futile; wars of aggression are specifically prohibited by Briand-Kellogg pact, but nowadays even aggressors try to dress the situation as justified defense and often get away with it; few wars since 1945 were tried by a competent tribunal and judged unlawful).

It isn't unlawful per se to cause civilian casualties during military operations; any demand that the warring parties limit themselves to killing combatants only would be unrealistic, especially in urban settings.

It is unlawful to target civilians intentionally or to cause wanton damage to civilian infrastructure, though.