Israel invaded Gaza due to the years long bombardment of its southern territory by terrorists in Gaza. Gaza, and the Palestinians in general, have been given more foreign aid per capita than any other group in the world, yet it has largely been squandered through corruption, violence, and inefficiency. Gaza has been given many opportunities, and instead insists on continuing to attack Israel - the invasion was the end result of such action.
Violence is never an acceptable means of conflict resolution, except in defense. It is often cited that Israel is defending itself, when attacking Gaza, however missiles for missiles is not self defense, but rather an offensive attack. Self defense would be directed attacks only hitting targets directly involved with attacking Israel. The bombing of Gaza is not justified. The invasion of Gaza is justified if it is to seize or kill combatants who have committed acts of war against Israel. Gaza has waged no war with Israel and should not be held responsible for the actions of individuals.