Since then, Israeli leaders have repeatedly sought to deny the Palestinians' national ambitions. When she was prime minister, Golda Meir famously remarked: "There is no such thing as a Palestinian." Pressure from extremist violence and Palestinian population growth has forced subsequent Israeli leaders to disengage from the Gaza Strip and consider other compromises, but not even Yitzhak Rabin was willing to offer the Palestinians a viable state. Ehud Barak's purportedly generous offer at Camp David would have given them only a disarmed set of Bantustans under de facto Israeli control.

The tragic history of the Jewish people does not obligate the US to help Israel today no matter what it does.

Israel's backers also portray it as a country that has sought peace at every turn and shown great restraint even when provoked. The Arabs, by contrast, are said to have acted with great wickedness. Yet Israel's record is not distinguishable from that of its opponents. The creation of Israel in 1947-48 involved ethnic cleansing, including executions, massacres and rapes by Jews, and Israel's subsequent conduct has often been brutal, belying any claim to moral superiority. From 1949 to 1956, for example, Israeli security forces killed 2700 to 5000 Arab infiltrators, most of them unarmed. The Israeli Defence Force murdered hundreds of Egyptian prisoners of war in the 1956 and 1967 wars; in 1967 it also expelled 100,000 to 260,000 Palestinians from the newly conquered West Bank and drove 80,000 Syrians from the Golan Heights.

The Palestinian resort to terrorism is wrong but it isn't surprising. Palestinians believe they have no other way to force Israeli concessions. So if neither strategic nor moral arguments can account for US support for Israel, how are we to explain it?

The explanation is the unmatched power of the Israel lobby. We use "the lobby" as shorthand for the loose coalition of individuals and organisations that work to steer US foreign policy in a pro-Israel direction. This is not meant to suggest that the lobby is a unified movement with a central leadership or that individuals within it do not disagree on certain issues. Not all Jewish Americans are part of the lobby because Israel is not a salient issue for many of them. In a 2004 survey, about 36per cent of American Jews said they were "not very" or "not at all" emotionally attached to Israel.

Jewish Americans also differ on specific policies. Many of the key organisations in the lobby, such as the American Israel Public Affairs Committee and the Conference of Presidents of Major American Jewish Organisations, are run by hardliners who generally support the Likud party's expansionist policies, including its hostility to the Oslo peace process. The bulk of US Jewry is more inclined to make concessions to the Palestinians and a few groups - such as Jewish Voice for Peace - strongly advocate such steps.

Despite these differences, moderates and hardliners both favour giving steadfast support to Israel. Jewish Americans have set up an impressive array of organisations to influence US foreign policy, of which AIPAC is the most powerful and best known. In 1997, Fortune magazine asked members of Congress and their staffs to list the most powerful lobbies in Washington. AIPAC was ranked second behind the American Association of Retired People but ahead of the AFL-CIO union movement and the National Rifle Association. A National Journal study in March last year reached a similar conclusion, placing AIPAC in second place (tied with AARP) in the Washington muscle rankings.

The lobby also includes prominent evangelical Christians such as Gary Bauer, Jerry Falwell, Ralph Reed and Pat Robertson, as well as Dick Armey and Tom DeLay, former majority leaders in the US House of Representatives, all of whom believe Israel's rebirth is the fulfilment of biblical prophecy and support its expansionist agenda; to do otherwise, they believe, is contrary to God's will. Neo-conservative gentiles such as UN ambassador John Bolton, former Wall Street Journal editor Robert Bartley, former US secretary of education William Bennett, former UN ambassador Jeane Kirkpatrick and columnist George Will are also steadfast supporters.

In its basic operations, the Israel lobby is no different from the farm lobby, steel or textile workers unions or other ethnic lobbies. There is nothing improper about American Jews and their Christian allies attempting to sway US policy; the lobby's activities are not a conspiracy of the sort depicted in tracts such as The Protocols of the Elders of Zion. For the most part, the individuals and groups that comprise it are doing what other special interest groups do, but doing it very much better. By contrast, pro-Arab interest groups, in so far as they exist at all, are weak, which makes the Israel lobby's task even easier.

The lobby pursues two broad strategies. First, it wields its significant influence in Washington, pressuring Congress and the executive branch. Whatever an individual legislator or policy-maker's views may be, the lobby tries to make supporting Israel the smart choice. Second, it strives to ensure that public discourse portrays Israel in a positive light, by repeating myths about its founding and by promoting its point of view in policy debates. The goal is to prevent critical comments from getting a fair hearing. Controlling the debate is essential to guaranteeing US support because a candid discussion of US-Israeli relations may lead Americans to favour a different policy.

AIPAC forms the core of the lobby's influence in the US Congress. Its success is due to its ability to reward legislators and congressional candidates who support its agenda and to punish those who challenge it. Money is critical to US elections (as the scandal over lobbyist Jack Abramoff's shady dealings reminds us), and AIPAC makes sure that its friends get strong financial support from the many pro-Israel political action committees. Anyone who is seen as hostile to Israel can be sure that AIPAC will direct campaign contributions to their political opponents. AIPAC also organises letter-writing campaigns and encourages newspaper editors to endorse pro-Israel candidates.

Thanks in part to the influence Jewish voters have on presidential elections, the lobby also has significant leverage over the executive branch. Although they make up less than 3 per cent of the population, they make large campaign donations to candidates from both parties. The Washington Post once estimated that Democratic Party presidential candidates depend on Jewish supporters to supply as much as 60 per cent of the money. And because Jewish voters have high turnout rates and are concentrated in key states such as California, Florida, Illinois, New York and Pennsylvania, presidential candidates go to great lengths not to antagonise them.

During the Clinton administration, Middle Eastern policy was largely shaped by officials with close ties to Israel or to prominent pro-Israel organisations, among them Australian-educated Martin Indyk, the former deputy director of research at AIPAC and co-founder of the pro-Israel Washington Institute for Near East Policy; Dennis Ross, who joined WINEP after leaving government in 2001; and Aaron Miller, who has lived in Israel and often visits the country. These men were among Bill Clinton's closest advisers at the Camp David summit in July 2000. The situation is even more pronounced in the Bush administration, whose ranks have included fervent advocates of the Israeli cause such as Elliot Abrams, Bolton, Douglas Feith, Lewis "Scooter" Libby, Richard Perle, Paul Wolfowitz and David Wurmser. These officials have consistently pushed for policies favoured by Israel and backed by organisations in the lobby.

Pressure from Israel and the lobby was not the only factor behind the decision to attack Iraq in March 2003 but it was critical. Some Americans believe that this was a war for oil, but there is hardly any direct evidence to support this claim. Instead, the war was motivated in good part by a desire to make Israel more secure. According to Philip Zelikow, a former member of the President's Foreign Intelligence Advisory Board and executive director of the 9/11 Commission, and a counsellor to US Secretary of State Condoleezza Rice, the real threat from Iraq was not a threat to the US. The unstated threat was the threat against Israel, Zelikow told an audience at the University of Virginia in September 2002.

Israeli leaders were deeply distressed when George W. Bush decided to seek Security Council authorisation for war and even more worried when Saddam Hussein agreed to let UN inspectors back in. "The campaign against Saddam Hussein is a must," Shimon Peres told reporters in September 2002. "Inspections and inspectors are good for decent people, but dishonest people can overcome easily inspections and inspectors."

Although neo-conservatives and other lobby leaders were eager to invade Iraq, the broader American Jewish community was not. Just after the war started, columnist Samuel Freedman reported that "a compilation of nationwide opinion polls by the Pew Research Centre shows that Jews are less supportive of the Iraq war than the population at large, 52per cent to 62 per cent". Clearly, it would be wrong to blame the war in Iraq on Jewish influence. Rather, it was due in large part to the lobby's influence, especially that of the neo-conservatives within it.

The neo-conservatives had been determined to topple Saddam even before Bush became President. They caused a stir early in 1998 by publishing two open letters to Clinton, calling for Saddam's removal from power. The signatories, many of whom had close ties to pro-Israel groups such as the Jewish Institute for National Security Affairs or WINEP, and who included Abrams, Bolton, Feith, Perle, Wolfowitz, William Kristol, Bernard Lewis and Donald Rumsfeld, had little trouble persuading the Clinton administration to adopt the general goal of ousting Saddam. But they were unable to sell a war to achieve that objective. They were no more able to generate enthusiasm for invading Iraq in the early months of the Bush administration. They needed help to achieve their aim. That help arrived with 9/11. Specifically, the events of that day led Bush and Dick Cheney to reverse course and become strong proponents of a preventive war.

One may argue that Israel and the lobby have not had much influence on policy towards Iran because the US has its own reasons for keeping Iran from going nuclear. There is some truth in this, but Iran's nuclear ambitions do not pose a direct threat to the US. If Washington could live with a nuclear Soviet Union, a nuclear China or even a nuclear North Korea, it can live with a nuclear Iran. And that is why the lobby must keep up constant pressure on politicians to confront Tehran. Iran and the US would hardly be allies if the lobby did not exist but US policy would be more temperate and preventive war would not be a serious option.

Can the lobby's power be curtailed? One would like to think so, given the Iraq debacle and the obvious need to rebuild America's image in the Arab and Islamic world.

Although the lobby remains a powerful force, the adverse effects of its influence are increasingly difficult to hide. Powerful states can maintain flawed policies for quite some time, but reality cannot be ignored forever. What is needed is a candid discussion of the lobby's influence and a more open debate about US interests in this vital region. Israel's wellbeing is one of those interests, but its continued occupation of the West Bank and its broader regional agenda are not. Open debate will expose the limits of the strategic and moral case for one-sided US support and could move the US to a position more consistent with its own national interest, with the interests of the other states in the region and with Israel's long-term interests as well.

London Review of Books

This is an edited extract from a longer article. John Mearsheimer is the Wendell Harrison professor of political science at the University of Chicago and author of The Tragedy of Great Power Politics. Stephen Walt is the Belfer professor of international affairs at Harvard University's Kennedy School of Government. His most recent book is Taming American Power: The Global Response to US Primacy.

End