A rising tide of public protest on the one hand, and a resumption of Palestinian mortar and rocket attacks on Jewish towns and villages in and near the Gaza Strip on the other hand: Even before it has physically started, Ariel Sharon’s disengagement-from-Gaza plan seems to be turning into a worse-case scenario nightmare.

The heavy spate of inaccurate but frightening homemade artillery barrages, largely the work of Hamas (which until now had adhered to the informal “cease-fire” worked out between the Palestinian Authority and Israel last summer), is the last thing Mr. Sharon wanted. It not only creates a major military headache for the army at a time when it needs to concentrate on the difficult task of evacuating Gaza’s Jewish settlers. Nor does it only evoke the specter of settlers being pulled from their homes next month while shells and missiles explode all around them, thus suggesting an ignominious retreat under fire. It threatens to yank the rug from under the very logic of disengagement itself.

Part of this logic, after all, has always been that pulling out of the Gaza Strip and freeing its Palestinian inhabitants of Israel’s presence would encourage them to turn their energies inward and put an end to Gaza’s use as a massive anti-Jewish terror base. Yet if at the very moment that Israel is leaving Gaza its Islamic militants rain down long-distance havoc, this does not bode well for the future.

At the very least, it means that more such attacks may occur after disengagement, too, if Hamas and its allies feel that it serves their purpose. This poses serious questions about the utility of disengagement in the eyes of its supporters at the a time when Ariel Sharon can least risk losing their support.

Moreover, to the extent that the Palestinians are able to go on lobbing self-manufactured ordnance at Israel over the security fence that has surrounded the Gaza Strip for years, they will also, probably sooner than later, be able to fire over the West Bank security fence that is scheduled for completion next year. An eventual pullback to this fence in the West Bank, which would appear to be Ariel Sharon’s long-term strategy, will be seriously jeopardized if it cannot be expected to stop mortar and rocket strikes.

Perhaps this is what Hamas, which has never hid its determination to destroy all of Israel and not just recover parts of it, has in mind. Whether Israel evacuates the Gaza Strip or most of the West Bank is not that crucial to Hamas, especially since it is not yet ready to vie for power with Arafat and Abu-Mazen’s Fatah in either area. Its leaders may very well calculate that if, under pressure from its citizens, Israel executes a last minute about-face and cancels the Gaza disengagement, their organization can only benefit.

Such an about-face will not take place. Yet Israel cannot afford to let rocket and mortar attacks from Gaza continue. Conceivably, it will refrain from a harsh reaction until disengagement is completed, in order not to escalate the situation even more. Afterward, though, one can expect it to react to such attacks with unprecedented force, causing heavy Palestinian casualties, until they stop. There will be no choice.

The settlers’ protests will not compel an about-face, either. Yet they, too, even though they will not stop disengagement from Gaza, call into question future withdrawals in the West Bank. If massive demonstrations, civil disobedience, and the threat of nationwide chaos are occasioned by the evacuation of 8,000 Jews from Gaza, a region of little historical or strategic importance, how can any Israeli government contemplate evacuating tens of thousands of Jews from the biblical heartland of Judea and Samaria?

The fact is that the Sharon government, like others before it, has only itself to blame for the threat of mass settler lawlessness. Although it is foolish to demonize the settler movement, as the Israeli left has long been doing, it is also foolish to idealize it. For years now, the settlers, who live in places where Israeli law does not fully apply and in which Israel’s police force is mostly noticeable by its absence, have gotten used to being a law unto themselves, doing what they want and how and where they want it. Rarely have the authorities made a serious attempt to curb them or to bring them to justice for infractions that no Israeli living in Israel proper could commit with impunity.

A year and a half ago, I wrote in this column, in discussing the unauthorized settler outposts that exist all over the West Bank, “No country that takes its legal system seriously can afford to see it flouted open and systematically as has been done by the wildcat land grabs that the outposts have involved.” Yet despite repeated promises on the government’s part, almost none of the outposts have been dismantled to this day. The message to the settlers has been clear: There is almost nothing they cannot get away with.

This is the message that the settler movement is taking with it in its campaign against disengagement from Gaza. In consequence, its confrontation with the government has turned into a struggle going far beyond the question of disengagement itself. It has become about whether, and by whom, the law will be obeyed in Israeli society. Much of the settler movement is unconvinced at this moment that the law applies to it, especially when it comes to the occupied territories.

Israel cannot afford to lose this summer’s battle with the settlers any more than it can afford to lose this summer’s battle with Hamas. The outcome of both will have repercussions far into the future.

Mr. Halkin is a contributing editor of The New York Sun.