Last year, I agreed to speak to a Jewish youth group about my organization, Jewish Voice for Peace, and our opposition to Israel's occupation. My talk was to follow one from a member of the American Israel Public Affairs Committee, which calls itself "America's pro-Israel Lobby."

A week before, a shaken program leader said the AIPAC staffer had threatened to get the entire youth program's funding canceled if I was allowed in the door. The threat worked, and in disgust, they canceled the whole talk.

Pundits will surely argue for years about professors Stephen Walt and John Mearsheimer's explosive new book, The Israel Lobby, which blames poor U.S. policy in the Middle East on a loose network of individuals and pro-Israel advocacy groups.

But the book, and the response to it, opens up another controversy: the stifling of debate about unconditional U.S. support for Israeli policies.

Why is Israel's increasingly brutal 40-year occupation of Palestinian land regularly debated in the mainstream media abroad, including in Israel, but not here? And why is there an almost total lack of discussion among presidential candidates about the dollars that subsidize this occupation and the American diplomatic support that makes it possible?

In a society built on the free exchange of ideas, as Walt and Mearsheimer point out, one answer can be found by looking at the many self-appointed gatekeepers, such as Abraham Foxman and the Anti-Defamation League, or Harvard law professor Alan Dershowitz, who use their Jewish identity as both a shield and cudgel. They work diligently to silence those who question ill-conceived policies of the Israeli and U.S. governments.

Non-Jewish critics, even former President Carter, are denounced as anti-Semites. Special ire is reserved for Jewish dissenters, who are branded as "self-hating" or "marginal," while Muslim and Arab-Americans are easily smeared and even criminalized with charges of supporting terrorism.

Stunned by the stifling of dissent, we decided to start a Web site, Muzzlewatch, to track the incidents. Just as we launched, Stanford Middle East Studies Professor Joel Beinin was disinvited from a speaking engagement at a high school with just 24 hours' notice.

After an unprecedented campaign of outside interference waged by Dershowitz, Professor Norman Finkelstein was refused tenure by DePaul University because of his criticism of U.S.-Israeli policy. Palestinian-American anthropologist Nadia Abu El-Haj is fighting a political campaign to deny her tenure at Barnard.

Even Walt and Mearsheimer, who are getting plenty of exposure, couldn't have asked for better proof of their point that the lobby works to stifle dissent when an embarrassed head of the Chicago Council on Global Affairs told them that their scheduled speech was canceled. (They did speak before the World Affairs Council of Dallas/Fort Worth on Sept. 17.) This was apparently because Foxman was not available that day to "balance" their talk.

(They had initially been booked by themselves. The talk was not rescheduled.)

Many groups that started with the important work of fighting real anti-Semitism now rely on anti-Semitism to insist that to show one's love of Jews, one must offer uncritical support to Israel. They are especially displeased by Jews who believe that enabling Israeli violations of Palestinian human rights is not good for anyone.

Unless this atmosphere of intimidation is confronted, Americans will continue to lack access to information and perspectives necessary to formulate effective Middle East policies, virtually ensuring that Israel and the United States will be at war for many years to come.