[Update, 11/21, 3:55 p.m.: The ceasefire announced today envisions "opening the crossings and facilitating the movement of people and transfer of goods..." Too soon to say what this will mean in practice.]

President Obama and Bibi Netanyahu are on the same page when it comes to the justification for Israel's bombardment of Gaza. Netanyahu : "No country in the world would agree to a situation in which its population lives under a constant missile threat." Obama: "There's no country on earth that would tolerate missiles raining down on its citizens from outside its borders."

It's true that if, say, Canada were lobbing missiles into the US, the US wouldn't tolerate it. But here's another thing the US wouldn't tolerate: If Canada imposed a crippling economic blockade, denying America the import of essential goods and hugely restricting American exports. That would be taken as an act of war, and America would if necessary respond with force--by, perhaps, lobbing missiles into Canada.

This is the situation Gaza has faced for years: a crippling economic blockade imposed by Israel. Under international pressure, Israel has relaxed the import restrictions, but even so such basic things as cement, gravel, and steel are prohibited from entering Gaza. The rationale is that these items are "dual use" and could be put to military ends. But this logic doesn't explain the most devastating part of the blockade--the severe restrictions on Gaza's exports.

Gazans can't export anything to anyone by sea or air, and there are extensive constraints on what they can export by land. They can't even sell things to their fellow Palestinians in the West Bank. According to the Israeli NGO Gisha, the number of truckloads of goods that leave Gaza each month is two percent of what it was before the blockade was imposed. (A black market trade via tunnels to Egypt has taken up some, but by no means all, of the slack.)

No wonder Gaza's unemployment rate has risen to 28 percent. No wonder 70 percent of Gazans receive humanitarian aid. No wonder there's a shortage of schools--it's hard to build them without construction materials.

If you mention the blockade to the average reasonably well-informed American or Israeli, you'll likely get a reply such as: Well, if the Gazans don't like economic strangulation, Hamas should quit firing missiles at Israel; or Hamas should recognize the state of Israel; or Hamas should do something else Israel wants it to do.

So, over the past couple of days, I tried to find out exactly what actions on the part of Hamas would suffice to end the blockade. And, after contacting various experts by email, I discovered that the answer is: nothing would suffice. At least, nothing we know of. Apparently Israel hasn't articulated clear conditions under which the blockade would end.

As law professor Noura Erakat has written in a journal article:

Despite claims of self-defense, Israel has not defined a definitive purpose for the blockade, the achievement of which would indicate its end. Official Israeli goals have ranged from limiting Hamas's access to weapons, to seeking retribution for the pain caused to Israeli civilians, and to compelling the Palestinian population to overthrow the Hamas government...

This seems kind of strange. I thought sanctions and blockades and the like were supposed to have specific purposes. The sanctions against South Africa, for example, would end when apartheid ended. So when will the blockade of Gaza end? If there's no answer, why should anyone expect the situation in Gaza to improve? If the Gazan people are being treated this harshly, and there's no end in sight, why does President Obama sound so surprised and outraged that violence against Israel would emanate from Gaza?

I'm not saying the blockade justifies the firing of missiles. And I'm not saying it doesn't--I'm just not getting into that messy issue right now, and I'd have to study up on international law before I did. But I'm saying that, when you subject people to treatment like this, without even specifying the conditions under which the treatment would change, human nature pretty much ensures that bad things, including violent ones, will happen.