Where I draw the line in this century is at deliberate or wanton attacks directed primarily at civilians, which attacks have little or no chance at greatly reducing casualties, and, particularly, where the purpose is terror of a civilian population as opposed to taking out a military target (including logistics.)
Wantonly spraying Israeli civilians with missiles is terrorism. Accurate attacks by Hamas on Israeli army bases would be fair game, in my view.
The problem of not attacking civilian populations and the just war theory associated with this idea is that civilian populations create the economic engine that funds the war machine. This is why, when a group of people (call it a country) are exetentially threatened, civilians are eventually targeted.
The US did this in WWII with the carpet bombing of dresden and the nuking of two japanese cities among just a few examples. There are many more throughout history.