Because the Radical Hamas (Spelling) terrorist group kept lobbing mortars, rockets and artillery into Israel intentionally killing hundreds of civilians for several weeks under a banner of a cease fire. The Israelis tried to negotiate for them to stop but they simply gave them the finger and just continued to bomb them. Israel has every right to defend them selves. There are nations such as France and Germany who disagree with Israel’s retaliatory strike but Israel did not start this, in fact any war involving Israel they never start, it's always these extremist Palestinian groups.
No one is going to win as another cease fire will be called for then they will fallow for several months then Hamas will start the fighting up again because they are A** holes and believe that Israelies don't deserve to live.