In a war between two countries, the decision to attack another country is governed by the UN Charter and the customary law of self-defence.  This is the branch of international law known by the Latin tag jus ad bellum, defining when the resort to armed force is justified.  The UN Charter recognizes the right of self-defence against an armed attack, and customary law requires that any war undertaken in self-defence be limited so that its extent is proportionate to the threat it is designed to counter.  Since Gaza is not an independent state, and Hamas not recognized as a legitimate government, there is some ambiguity about how these rules would apply in this case.  Nevertheless there is wide agreement that the repeated launching of rockets by Palestinian militants against Israel would count as a legitimate reason for Israeli action.
http://crimesofwar.org/onnews/news-gaza3.html