Yuval Steinitz, a prominent Likud member of the Israeli Knesset and the chairman of the Knesset's Foreign Affairs & Defense Committee, admits that Israel has been planning an attack on the people of Gaza for at least eight to nine months.
This goes a long way to explain why Israel suddenly attacked, despite the fact that there were no Hamas rocket attacks during the ceasefire, until Israel violated the ceasefire on November 5th.
Israel went forward with the planned attack on Gaza, even though Hamas rocket attacks after the end of the ceasefire failed to kill any Israelis, and despite a Hamas temporary halt in rocket attacks and proposed renewal of the ceasefire -- which Israel refused -- in mid-December.
In other words, this attack was most likely a premeditated attempt for Israel to weaken Hamas -- which was elected into power with a 56% majority of the Palestinian people -- and strengthen the unelected Palestinian Authority. It appears, however, that they've utterly failed to achieve these goals, both amongst the Palestinians and abroad.
Keep in mind that in early 2008, Israel asked the U.S. for permission to attack Iran with US bunker buster weapons, and were turned down because such attacks were considered unlikely to be effective. In a very real sense, attacking Gaza may be an effort to rebuild Israeli confidence in the use of military power after they were forced to withdraw from Lebanon in 2006.
Unfortunately, this goal appears to have succeeded, at least in the short term. Israel's politicians have surged in popularity since the start of the invasion, in part because the humanitarian toll is not being covered by Israel's media. Israel continues to bar the world's journalists and relief workers from going into Gaza, and Israel's major newspapers are not visibly reporting on Palestinian casualty figures, as most Western democracies are doing.