Late this past June, a group of Israeli settlers in the West Bank defaced and burned a mosque in the small West Bank village of Jabaa. Graffiti sprayed by the vandals warned of a “war” over the planned evacuation, ordered by the Israeli Supreme Court, of a handful of houses illegally built on private Palestinian land near the settlement of Beit El. The torching of the mosque was part of a wider trend of routine violence committed by radical settlers against innocent Palestinians, Israeli security personnel, and even mainstream settler leaders — all aimed at intimidating perceived enemies of the settlement project.

In the past, settlers who opposed attacks against Palestinian civilians or the Israeli state (the vast majority of them) could exert control over radical elements, or work with the Israeli authorities to do so. Recently, however, several factors have contributed to a rise in unchecked settler radicalism: the dramatic growth over the past generation in the size of the settler population, the diversification of religious and ideological strands within it, and the sense of betrayal felt by settlers following Israel’s evacuation of the settlements in the Gaza Strip in 2005.

Israel, through the Israel Defense Forces and other security agencies, must now assert control over groups that no longer respect the state or the traditional settler leadership.

Although radical settler violence poses an increasing threat, mainstream Israeli society appears ever more apathetic to the fate of the Palestinians. Negotiations between Israel and the Palestinians remain deadlocked, and even their meaningful resumption, let alone success, seems unlikely in the near future. The Israeli government thus feels little pressure to confront the extremists.

But with the peace process frozen, what happens under Israeli control matters more, not less. With Israel likely to govern parts of the West Bank for some time, it can no longer shirk its obligations — to protect not only its own citizens but also Palestinian civilians under its control — by claiming that a two-state solution is on the horizon and that the Palestinians will soon assume full responsibility over themselves. And if Israel wants to preserve the possibility of a negotiated peace, it must address this problem before it is too late.

Whenever extremist settlers destroy Palestinian property or deface a mosque, they strengthen Palestinian radicals at the expense of moderates, undermining support for an agreement and delaying a possible accord. Meanwhile, each time the Israeli leadership caves in to the demands of radical settlers it vindicates their tactics and encourages ever more brazen behavior, deepening the government’s paralysis. In other words, Israeli violence in the West Bank both undermines the ability of Israel to implement a potential deal with the Palestinians and raises questions about whether it can enforce its own laws at home.

Recently, Israeli leaders have begun to recognize the problem. Following extremist vandalism against the I.D.F. and mainstream settler leaders over the past year, some Israeli generals and government ministers identified radical violence as terrorism. The label might seem hyperbolic but such low-level violence does deserve to be called “terrorism.” The carnage does not compare to suicide bombings but it is political in purpose, perpetrated by a nonstate actor, directed against non-combatants, and intended to intimidate and scare both the Israeli government and ordinary Palestinians — these factors meet academic and government definitions of the word.

Now, the Israeli government should translate that bold rhetoric into decisive action. To begin with, it should officially designate the perpetrators of this violence as terrorists and disrupt their activities more aggressively. Security agencies should then enforce Israeli law, prosecuting violent settlers as they would terrorists, Palestinian or Israeli. And to slow the tide of radicalism, Israeli leaders must denounce extremists and shun their representatives, placing particular pressure on religious leaders who incite violent extremism.

Meanwhile, the United States and other countries seeking to revive the peace talks must encourage Israel to take these steps before the situation gets worse. Washington has long hoped that issues related to settler violence would vanish after the implementation of a peace deal. In the absence of meaningful negotiations, however, it must prevent both parties from deepening tensions. By highlighting the problem of settler extremism, the United States can push Israel into responding to it more effectively. The United States, like Israel, should consider designating violent radical settlers as terrorists. Such a designation would allow U.S. authorities to prevent Americans from sending them funding and would be a way to support those Israelis seeking to combat the rise of extremism.

Almost everything related to the Israeli-Palestinian dispute involves complex trade-offs. But trying to stop extremist violence is a clear moral and political imperative. Israelis, who know the horrors of terrorism better than the citizens of any other democracy, should have a special understanding of the need to ensure that extremist violence does not ruin the lives of Palestinians and prevent Israel from making hard but necessary choices about its future. Whether the conflict continues indefinitely or the peace talks resume, Israel must confront its homegrown terrorism problem.

Extremist violence tarnishes Israel’s name and imperils its future. Friends of Israel, the Israeli government, and even those who support the settlements in the West Bank should fight back against this dangerous phenomenon.