It’s rare for Israel’s left and right flanks to agree on anything. But last week, the party leaders of left-wing Meretz and right-wing Otzma LeYisrael were in full accord: Both declared that Hamas cannot be deterred.Aryeh Eldad of Otzma LeYisrael devoted an entire op-ed to asserting that fanatic Jew-haters aren’t deterrable, so “The hope that our tremendous military power will deter our enemies ... must be revised.” Zahava Gal-On of Meretz espoused the same conclusion for opposite reasons: Only diplomacy can solve problems, she opined, so force can’t deter Hamas; "Deterrence is an illusion held by the attacker, but it has no basis in the behavior of the attacked.”Yet while Hamas is certainly undeterred right now, Israel has deterred other enemies in the past. Thus instead of dismissing deterrence as unworkable, it’s worth asking what conditions produced past successes and whether and how they could be reproduced against Hamas. A good starting point is Eldad’s lament that even Israel’s “greatest victory ever,” the 1967 Six-Day War, didn’t prevent Egypt and Syria from launching the War of Attrition “just days after the fighting ended” or the Yom Kippur War six years later. “If a decisive defeat such as the Six-Day War did not deter them, what would?” he demanded.The answer, as Eldad should know, is the Yom Kippur War. In its first 25 years of existence, Israel fought four full-scale conventional wars. But in the 40 years since 1973, no Arab country has launched or even seriously threatened Israel with a conventional war. And that isn’t because they suddenly stopped hating Israel. It’s because Egypt, Syria and Jordan all became convinced that a) they couldn’t beat it and b) trying would exact a painful price.The Six-Day War alone couldn’t do this because despite losing sizable chunks of territory, Arab countries could still persuade themselves it was a fluke: It might have ended very differently if Israel, facing three armies massed on its borders, hadn’t launched a preemptive strike on Egypt’s air force. In 1973, in contrast, Egypt and Syria achieved complete surprise: Attacking on the holiest day of the Jewish year, they inflicted heavy casualties and forced Israel into retreat. Yet the war still ended with Israeli troops threatening both Cairo and Damascus. And the fact that the Arabs lost even under optimal opening conditions finally convinced them that Israel couldn’t be beaten.No less important, however, was the territorial price these wars exacted. The world didn’t pressure Israel into returning the territory captured in 1967; UN Security Council Resolution 242 was explicitly worded to let Israel keep some of this territory and return the rest only in exchange for full peace. Nor did the Arab aggression in 1973 produce Israeli concessions. Only by becoming the first Arab country to sign a peace treaty with Israel did Egypt finally recover the Sinai. These, then, are the components of successful deterrence: The enemy must be convinced both that it cannot win and that fighting will exact a price. And neither is currently true of Hamas. Partly, this is the “international community’s” fault. It’s Israel, not Hamas, that’s being condemned worldwide for a war Hamas started. It’s Israel, not Hamas, that’s being threatened with boycotts and indictment in international courts. And it’s Israel, not Hamas, from which the world is demanding concessions: The West has been pressuring Israel to end the Gaza blockade, which is Hamas’s key demand, in exchange for nothing but a vague “international monitoring mechanism” that won’t prevent Hamas from rearming, while relegating Israel’s demand that Hamas disarm to the distant future of an Israeli-Palestinian peace agreement. All this leads Hamas to conclude that aggression will reap gains while costing it nothing it cares about. After all, it doesn’t care how much Gaza’s civilians suffer; indeed, from its perspective, the more, the better, since civilian suffering increases international pressure on Israel. But Israel’s own behavior is equally to blame for Hamas’s conclusion. As examples, consider the following two news items. Last Friday, a prominent newspaper’s military analyst declared that quiet won’t return “as long as Hamas feels beaten and humiliated,” so “If Israel wants quiet from Hamas, it needs to give it that ‘something’ that will allow it to tell the battered people of Gaza that the war was worth it.” The far-left Haaretz? No, that’s Yoav Limor of the “right-wing” Israel Hayom. Eleven days earlier, another prominent newspaper’s military analyst warned that further fighting is almost inevitable, because Israel “has not been displaying any determination in the cease-fire negotiations, but instead is conveying a desire to be done with all this Gaza stuff and return to routine,” while “the Israel Defense Forces reduced its troops around Gaza too quickly, so Hamas doesn’t feel any real threat that Israel will reenter the Strip.” Some right-wing rag? No, that’s Amos Harel of Haaretz. Limor’s article exemplifies one major problem: Even on the center-right, many opinion leaders – including key ministers – favor buying (temporary) quiet with diplomatic concessions. Consequently, the government reportedly offered numerous concessions during the failed cease-fire talks. Harel’s article reflects the second major problem: When even the left perceives a “right-wing” government as palpably desperate to end the fighting, Hamas doesn’t feel its vital interests – survival and control of Gaza – are under any threat. Thus for Hamas, attacking Israel is a triple win: no substantial costs, a real prospect of gains, and greater global hatred of Israel as a bonus. And as long as that remains true, deterring it will indeed be impossible.Israel can’t do much about the world’s response, but the other two elements are entirely in its control. Not only could it refuse to reward Hamas’s aggression, but it could make Hamas pay a territorial price – for instance, by reoccupying parts of Gaza near the border. That would show Hamas that aggression entails real costs, while also vastly enhancing the South’s security, since most mortars and short-range rockets are fired from these areas.It might fail, but it’s worth a try. Because the alternatives to deterring Hamas are far more costly: either destroy it completely, or live forever at its nonexistent mercy.