... who is the war criminal?
I think that the answer, in your hypothetical example, as well as the current Gaza conflict (and the previous three, actually), is both.
The tragedy is nothing will come of it. A UN report will determine that both sides committed war crimes. Israel will condemn this as anti-semitic, and Hamas will condemn it as depriving them of the only way they have left to resist Israeli military and economic warfare.
Even looking at root causes is futile, for a conflict this old. So an apparently simple question such as "who broke the ceasefire / truce?". For example, one might argue that it was Hamas, because they fired rockets from Gaza, before the first Israeli airstrike hit. But another might argue Israel, because it never even started implementing the conditions upon which the truce was achieved (settlement freeze, lifting the blockade, ...)
In any other case, the UN would send in peackeepers. But of course that is not possible here, because of the US' reflexive support for Israel (which, according to some, amounts to US legislators' mortal fear of AIPAC).