At a time when Israel is bombing Gaza to try to smash Hamas, it’s worth remembering that Israel itself helped nurture Hamas.

When Hamas was founded in 1987, Israel was mostly concerned with Yasser Arafat’s Fatah movement and figured that a religious Palestinian organization would help undermine Fatah. Israel calculated that all those Muslim fundamentalists would spend their time praying in the mosques, so it cracked down on Fatah and allowed Hamas to rise as a counterforce.

What we’re seeing in the Middle East is the Boomerang Syndrome. Arab terrorism built support for right-wing Israeli politicians, who took harsh actions against Palestinians, who responded with more terrorism, and so on. Extremists on each side sustain the other, and the excessive Israeli ground assault in Gaza is likely to create more terrorists in the long run.

If this pattern continues, we may eventually see Hamas-style Palestinians facing off against hard-line Israelis, with each side making the others’ lives wretched — and political moderates in the Middle East politically eviscerated.

I visited Gaza last summer and found many Palestinians ambivalent in a way that Americans and Israelis often don’t appreciate. Many Gazans scorn Fatah as corrupt and incompetent, and they dislike Hamas’s overzealousness and repression. But when they are suffering and humiliated, they find it emotionally satisfying to see Hamas fighting back.

Granted, Israel was profoundly provoked in this case. Israel sought an extension of its cease-fire with Hamas, and Egypt offered to mediate one — but Hamas refused. When it is shelled by its neighbor, Israel has to do something.

But Israel’s right to do something doesn’t mean it has the right to do anything. Since the shelling from Gaza started in 2001, 20 Israeli civilians have been killed by rockets or mortars, according to a tabulation by Israeli human rights groups. That doesn’t justify an all-out ground invasion that has killed more than 660 people (it’s difficult to know how many are militants and how many are civilians).

So what could Israel have reasonably done? Bombing the tunnels through which Gazans smuggle weapons would have been a proportionate response, if Israel had stopped there, and the same is true of airstrikes on certain Hamas targets. An even better approach would have been to ease the siege in Gaza, perhaps creating an environment in which Hamas would have extended the cease-fire. It was certainly worth trying — and almost anything would be better than lashing out in a way that would create more boomerangs.

“This policy is not strengthening Israel,” notes Sari Bashi, the executive director of Gisha, an Israeli human rights group that works on Gaza issues. “The trauma that 1.5 million people have been undergoing in Gaza is going to have long-term effects for our ability to live together.

“My colleague in Gaza works for an Israeli organization. She’s learning Hebrew, and she’s just the kind of person we can build a future with. And her 6-year-old nephew, every time a bomb drops from the air, is at first scared and then says — hopefully — maybe the Qassam Brigades will now fire rockets at the Israelis.”

Israel’s strategy has been to make ordinary Palestinians suffer in hopes of creating ill will toward Hamas. That’s why, beginning in 2007, Israel cut back fuel shipments for Gaza utilities — and why today, in the aftermath of the bombings, 800,000 Gaza residents lack running water, Ms. Bashi said.

“The Israeli policy on Gaza has been marketed as a policy against Hamas, but in reality it’s a policy against a million-and-a-half people in Gaza,” she said.

We all know that the most plausible solution to the Middle East mess is a two-state solution along the lines that former President Bill Clinton has proposed. It’s difficult to tell how we get there from here, but a crucial step is to strengthen President Mahmoud Abbas and his Palestinian Authority.

Instead, initial reports are that the assault on Gaza is focusing Arab anger on Mr. Abbas and moderate neighbors like Jordan, undermining the peacemakers.

My courageous Times colleague in Gaza, Taghreed el-Khodary, quoted a 37-year-old father weeping over the corpse of his 11-year-old daughter: “From now on, I am Hamas. I choose resistance.”

Barack Obama has said relatively little about Gaza. At first, given the provocations by Hamas, that was understandable. But as the ground invasion costs more lives, he needs to join European leaders in calling for a new cease-fire on all sides — and after he assumes the presidency, he must provide real leadership that the world craves.

Aaron David Miller, a longtime Middle East peace negotiator for the United States, suggests in his excellent new book, “The Much Too Promised Land,” that presidents should offer Israel “love, but tough love.”

So, Mr. Obama, find your voice. Fall in tough love with Israel.