Depending on who you ask, Israel or Gaza can be responsible.

Gazan Perspective: It's Israel's Fault

Israel broke the peace treaty and subsequently launched operation 'Cast Lead' a massive military offensive against a civilian population. It banned foreign press from entering Gaza. It used white phosphorus (a chemical weapon) on UN sanctioned hospitals and schools (a war crime). Within 23 days the death toll had reached 1,284 Palestinians dead, 6 IDF by enemy fire and 3 IDF by friendly fire. When the operation had finished Israel had claimed more land from the Gaza strip in the name of security zones for their protection.

Israeli Perspective: It's Hamas' Fault

Hamas failed to control both its own militants and those of the group Islamic Jihad to abide by the terms of the hudna (ceasefire) that they had negotiated with Israel and did not seem interested in renegotiating. They fired large volleys of rockets repeatedly into Israeli territory. After growing Israeli resentment over the attacks, the Israeli government saw fit to use a military incursion to stop the rocket bombardments.