Why did Israel attack Gaza?

Well its really hard to say why they attacked. But I myself personally believe a ceasefire should immediately be called. Prior to the conflict, both sides have launched missiles at each other and in a sense, both countries started the war. But its really bad for Gaza, they have a much smaller fighting force, the Hamas, and are weaker compared to the Israel, who outnumbers them. Its sad because many civilians in Gaza are being killed, I think more civilians are dying rather than people, which I find ridiculous. Many in Gaza are without electricity, and its hard for foreign nations to send aid when shelling and air strikes are an everyday occurrence. I hope the UN or some country does get involved with calling a ceasefire. I particularly think the US should step up on this issue since Israel and the US have very close ties. If this Israel-Gaza conflict continues, I think it will go out of hand, especially since many neighboring countries are going against Israel's decision to go on a full-scale attack. However, few countries in the middle east would actually never step up to fight Israel. Mainly because of their past, for example in the six-day war. In the six-day war, Israel fought against four other nations, and not only defeats those nations in a short amount of time, but they gain land. Along with Israel being a powerful nation, although small, the US is one of Israel's greatest allies