In its defence, Israeli spokespeople have claimed that the attack was a response to 'more than 100 rockets being fired in the previous 72 hours' (or some iteration thereof; you'll be hearing versions of this claim incessantly for the next few days). True or not, this aggregate misleads: it doesn't mention Israel's attacks over the same time period, of course, still less Israel's decades long occupation, which can only be sustained with constant low-level and periodic escalations of violence of the kind we're seeing now. But more narrowly: it conceals the fact that, prior to Israel's attack, Hamas and other Palestinian factions had offered, and largely adhered to, an informal truce.
Here, as best as I can gather from news sources, is the accurate chronology, focusing on Palestinian rockets only:
12 Nov: Palestinian militant groups announce that they've agreed to adhere to a truce, should Israel halt its operations. Islamic Jihad: 'The ball is in Israel's court'. There was 'no immediate response' from Israel, although Israeli Defence Minister Ehud Barak struck a belligerent tone with reporters, declaring: "The matter has definitely not ended". While Hamas took part in some of the missile launches at the weekend, by Monday it had stopped, 'suggesting it was looking to step back from the brink'. But already, Netanyahu had convened foreign ambassadors to prepare the diplomatic ground for an escalation.
12 Nov: According to the Israeli military, 12 rockets were fired on Nov 12; however Israel apparently later revised this number down to 6 rockets. This was clearly a 'significant drop' from the previous days: given that Al Jazeera reports that 115 rockets had been fired since Nov 10 (The Israeli military's estimate was 119, though this may have been before they revised down the 12 Nov figure). The rockets on 12 Nov were claimed not by Hamas but by smaller groups.
13 Nov: By 8am GMT, only 1 rocket had reportedly struck Israel. Israel, however, struck 3 targets in Gaza in the early hours of Tuesday (no casualties). [Al Jazeera] Two more rockets were fired into Israel in the afternoon. I couldn't find reference to any others. Israel and Hamas reportedly agreed a tacit truce, mediated by Egypt. Likud MK Benny Begin: "This round of firing appears to have ended".
14 Nov: Four rocket strikes reported.
14 Nov: Israel launches major attack, assassinating Jabari and striking over 20 locations across Gaza. The attack is still continuing. BBC, Reuters, the Jerusalem Post, the Guardian and Ynet all report that Israel's strike broke a lull in the fighting, during which rocket attacked had 'trickled down to a small number' (Jerusalem Post).
So: Israel was offered a truce; didn't respond; then tacitly agreed to a lull, which appeared to be working, until Israel shattered it with a massive escalation; in self-defence.
Past data suggests that Palestinian rocket fire largely follows rather than provokes Israeli violence, and that, once a lull or truce has been agreed, Israel overwhelmingly kills first. This latest escalation, which may expand still further, appears to have followed the same pattern.