A crisis is unfolding in the Middle East, and as expected Israel mounted a huge assault overnight and Gaza militants continued to launch rockets. A key development: the sound of air-raid sirens in Jerusalem and Tel Aviv. 

At least 20 Palestinians and three Israelis have been killed since Israel began its offensive on Wednesday. And the Israeli army has begun an initial draft of 16,000 reservists after the government authorized a call-up of 30,000. This is an indication, perhaps, that we could be at beginning of a protracted military exchange.

Natan Sachs researches Israeli foreign and domestic policy at the Saban Center for Middle East Policy at the Brookings Institute in D.C.

"Israel is preparing for the possibility of even a ground invasion," Sashs says, though he adds that this is not in their interest. "Israel would prefer to stop at this point — its scored a tactical victory from its point of view, in the killing of the head of the Hamas military operation." 

"But on the Hamas side, we've already seen attacks this morning — missiles that hit Tel Aviv," he says. 

"In recent months, the world press has not paid all that much attention, but Israelis have focused very strongly on the rocket fire on Israel," Sachs says. Many Palestinians — and indeed, some Israelis — fear that Netanyahu's actions are motivated by politics.

"Strategically, this could be quite dangerous in terms of relations between Israel and Egypt," Sachs says.