FIFTY-FIVE years ago, when Mussolini's troops overran Ethiopia, half-hearted sanctions by the League of Nations failed to force Italy to withdraw. Haile Selassie's futile pleas for help have haunted the world ever since.

This week, President Bush and key members of his administration including the secretaries of state and defense declared that the United Nations' far stronger sanctions against Iraq cannot be relied on to force a withdrawal from Kuwait. Only military power, they warned, is certain to get Saddam Hussein's armies out.

But sanctions can work -- and under circumstances far less favorable than those present in the confrontation with Iraq. In fact, a review of 115 cases since 1914 shows that success was achieved 40 times when economic sanctions were threatened or imposed against individual countries. Moreover, the current U.N. sanctions are by far the strongest and most complete ever imposed against any country by other nations. These comparisons strongly suggest that, given time, the U.N. economic boycott can achieve by peaceful means what Bush and his advisers say can only be won by force.

A comparison with the famous case of Ethiopia, one of the 115 we have reviewed in detail, reveals important differences which apply in the current case. The embargo of Iraq is completely different from the League's half-hearted attempt to save Ethiopia (which was made even weaker when the United States, a non-League member, refused to join). The current boycott covers virtually 100 percent of Iraq's trade. This is three to four times greater coverage than the average in all previous successful sanctions cases. Beyond that, Iraq, geographically isolated and dependent on oil for 90 percent of its export revenue, is far more vulnerable to economic coercion than target nations in other sanctions actions.

Because of all these factors, it is likely that if the embargo persists, Iraqi output will shrink by about half from its 1988 total of $45 billion. This is a decline of gross national product (GNP) 20 times greater than the average impact in other successful sanction episodes. Meanwhile, the economic costs to the sanctioning countries of suspended trade with Iraq are being addressed in unusual ways and substantially mitigated.

These efforts give the current sanctions a cohesion and possible longevity never seen outside the setting of global conflicts.

In addition, the administration's toughening military posture can have a welcome side effect: Such bellicosity could actually work to strengthen the resolve of the sanctioning nations to stick to their embargo as the only alternative to armed conflict. Economic sanctions have been used in this century in pursuit of a wide variety of goals. They range from the relatively modest, such as Britain's 1933 sanctions against the Soviet Union to gain the release of some British citizens accused of spying, to the difficult, such as the U.S. sanctions against Poland from 1981 to 1987 to force the communist regime to lift martial law and loosen political restraints.

In judging whether the imposition of sanctions was a "success," we looked for evidence of two things: that the boycotters had substantially met their goals; and that sanctions had contributed at least modestly to the outcome. Successful actions include, for example, the trade embargoes and financial sanctions to weaken the enemy's fighting capability used by the Allies in World Wars I and II and by Great Britain and its allies during the Falklands conflict in 1982. On two occasions in the 1920s, the mere threat of sanctions by the League of Nations was sufficient to settle border conflicts: Yugoslavia withdrew troops from disputed territory in Albania; Greece renounced territorial claims in Bulgaria. In the postwar era, the protracted U.N. embargo of Rhodesia, much less stringently enforced than the sanctions against Iraq, helped bring about the demise of the breakaway regime of Ian Smith.

Such examples argue strongly for the likely success of the sanctions against Iraq. Secretary of Defense Richard B. Cheney himself said the embargo "clearly" has been effective "in closing off the flow of spare parts and military supplies," and the chairman of the Joint Chiefs of Staff, Gen. Colin Powell, conceded that sanctions would have "a debilitating effect" on Iraq's military capability. On Thursday, CIA Director William H. Webster told the House Armed Services Committee that by next spring, "probably only energy-related and some military industries will be fully functioning."

The sanctions against Iraq are unique in the history of such economic weapons in the 20th century. Though there is inevitably some leakage, the embargo affects virtually all of Iraq's trade and financial relations. Historically, when the sanctioning country or group accounted for 50 percent or more of the target's trade, the sanctioners had a 50 percent chance of achieving their goals. In the average successful sanctions case, the boycotters accounted for 28 percent of the target's trade, far below the Iraq situation.

In addition, this embargo is backed by a multinational naval blockade and a ban on air cargo to Iraq. Except for what we consider minor smuggling through Turkey, Iran and Jordan, Iraq has been effectively isolated from the global economy. Smuggling will ebb as Saddam runs out of money, which Webster predicted would be next spring or summer.

The average cost to the target nation's economy in successful sanctions cases was 2.4 percent of GNP, about the level of lost U.S. output in the 1982 recession, (the most severe since the Depression), and one-twentieth of the impact on Iraq. The cost to the target reached double digits only three other times: Nigeria vs. Biafra, 1967-70; U.S. and Britain vs. Iran, 1951-53; and the U.N. and Britain vs. Rhodesia, 1965-79. In all these cases, sanctions contributed to a positive outcome. Of eight sanctions episodes where the cost to the target was 5 percent of GNP or more, six resulted in at least partial success for the sanctioners.

Prior to this summer, only Ian Smith's unilateral declaration of independence in Rhodesia in 1965 had provoked mandatory, comprehensive U.N. sanctions. However, those sanctions were imposed incrementally over two years and were not universally enforced despite being mandatory. Unlike the Iraq case, the U.N. refused to impose secondary sanctions against countries violating the Rhodesian embargo. The sanctions against Iraq were imposed so swiftly, decisively, and comprehensively that together with a credible military threat, there is a high probability they can contribute to an Iraqi withdrawal and the restoration of an independent government in Kuwait. However, our study of sanctions cases indicated that the more difficult the goals, the less effective the sanctions.

Besides the goals outlined in the U.N. resolutions, Bush and other leaders have talked of reducing Iraq's military capability, including the destruction of its nascent nuclear weapons capability. While sanctions can weaken Saddam's fighting capability because of food, fuel and spare parts shortages and resupply problems, they cannot destroy his arms industry.

There also have been suggestions that the sanctions should be aimed at destabilizing Saddam. The United States has taken this route before -- no less than 10 times since World War II. In fact, the United States far exceeds all other countries in threatening or using sanctions -- 81 attempts since 1917, of which more than 70 came after World War II. U.S. goals have varied widely -- from curbing or destabilizing governments perceived to be drifting from the "Western" capitalist sphere, to forcing Britain and France in 1956 to withdraw their troops from the Suez Canal after Egypt's Gamal Nasser nationalized it. In the 1970s, the United States increased its use of sanctions, not as successfully, to improve the observance of human rights and to inhibit the spread of nuclear weapons. In the 1980s, terrorism and drug-smuggling have been major targets of U.S. sanctions.

In the 10 cases of U.S. sanctions aimed at dictators, they contributed at least modestly to the downfalls of Rafael Trujillo in the Dominican Republic in the 1960s and Idi Amin in Uganda and Anastasio Somoza in Nicaragua in the 1970s. Sanctions also exacerbated the economic chaos in Nicaragua, which contributed to the electoral defeat of Daniel Ortega earlier this year.

In cases in which the goals were ambitious, sanctions took an average of nearly two years to achieve a successful outcome. This raises the question of their sustainability. Here again, the Iraq case is unique. To counter possible erosion of the boycott because the participants find the costs to their own economies too high, the United States and its allies have taken extraordinary steps, including asking Saudi Arabia and other oil exporters to boost oil production to offset lost Iraqi and Kuwaiti production. The United States also led in organizing an "economic action plan" to redirect short-term windfall profits gained by the oil producers to help developing countries. Washington also has encouraged Japan, Germany and others to provide grants and low-cost loans to developing countries hurt by higher oil prices, lost trade and related problems.

Maintaining a cohesive alliance long enough to make the sanctions work will require continued cost-reducing measures, such as getting the gulf oil producers to raise oil production so that prices come down and stabilize around the July OPEC target price of $21 per barrel. The United States, Germany and Japan also should be prepared to release oil from their strategic petroleum reserves to prevent price rises when winter brings increased energy consumption. The $21 billion committed to the economic action plan also should be swiftly distributed to offset costs to the front-line coalition states and further supplemented by additional grants for as long as needed to permit the sanctions to work. The IMF and World Bank should also increase concessional loans to developing countries thrown off balance by the sudden increase in oil prices.

However, even the tightest sanctions take time to work. Evidence from previous cases suggests that it would be unfair to claim the embargo of Iraq has failed until at least a year has passed. Though there are costs to waiting, some of them can be ameliorated, as with the president's economic action plan. If after a year or two the sanctions are judged to be inadequate, the military option will still be there and Saddam's forces will be weakened by lack of supplies. The key question is whether the price of patience would be higher than the economic and human costs of going to war soon.

Kimberly Elliott is a research associate and Jeffrey Schott is a research fellow at the Institute for International Economics. Gary Hufbauer is a visiting fellow at the institute and teaches international financial diplomacy at Georgetown University.