Clock ticking for Israeli decision makers
By last night, Israel had launched about 1,200 air strikes against targets across Gaza, killing 121 Palestinians, while Hamas had fired more than 500 rockets at Israeli cities, wounding 10 people.
Once, Israeli towns lying within a 25-mile radius of Gaza were most at risk; yesterday, Hamas was launching its longest-range weapons and sirens were sounding as far away as Haifa, almost 100 miles north.
Operation Protective Edge is still young by the standards of previous offensives. Today will be day six of the air strikes, whereas Operation Pillar of Defence lasted for eight days in 2012 and Operation Cast Lead went on for 22 days in 2008-09.
Several factors will determine the length of this ordeal. The first is the flow of Hamas rockets: until and unless this diminishes, Israel will be viscerally unwilling to scale down its offensive.
Before this operation began, Hamas was believed to possess a stockpile of up to 10,000 projectiles of various kinds.
Israel's decision-makers also will have to weigh international pressure for a ceasefire. So far, the diplomatic spotlight has been trained well away from them. John Kerry, the US secretary of state, waited until day three of the operation before making a bland call for "de-escalation".
On Friday, President Barack Obama spoke of mediating a "ceasefire" during a telephone conversation with Benjamin Netanyahu, the Israeli prime minister.
So far, America has not wavered from its traditional willingness to provide diplomatic cover for Israel. But experience suggests that international patience has limits and, at some point, even America's forbearance will crack. The blunt truth is that a large number of civilians usually have to die first.
Israeli decision-makers know the clock is ticking. Their military planners assume that all operations will eventually be curtailed by outside pressure, so they try to ensure that a large number of targets are destroyed as quickly as possible. Hence the intensity of an air campaign that has seen 1,200 strikes in only five days. In the coming days, they will consider whether to escalate the offensive by sending troops into Gaza.
Mr Netanyahu is a cautious leader. He will be reluctant to incur the risks of a ground operation. If he orders an invasion of Gaza, the casualties on both sides would inevitably mount and one terrible incident might jeopardise even America's goodwill.
The chances are that Israeli soldiers will strike inside Gaza only if there is no other way of achieving Mr Netanyahu's objective. And that aim is more limited than it might appear.
Put simply, he will trade "quiet for quiet". That leaves Hamas with the most agonising dilemma of all. How much more of its painstakingly amassed arsenal can it afford to fire at its old enemy - and how much retribution is it willing to take?