Economic sanctions have been called wars without bullets, yet their record in forcing political change has been mixed _ by some estimates successful about a third of the time.

Nonetheless, sanctions have grown in popularity since the end of the Cold War as an alternative to armed conflict. They enable governments to take a stand without placing their soldiers in harm's way.

In Syria, where opposition activists say a 17-month uprising against President Bashar Assad has claimed more than 20,000 lives, Libya-style military intervention is not in the cards. So the United States and its Western allies have turned to economic sanctions as a way to promote political change.

As in shooting wars, however, events don't always go according to plan.

Sanctions sometimes backfire, deepening the suffering of ordinary people, prolonging a conflict by stiffening the resolve of autocratic governments or even triggering war, as when the U.S., Britain and the Netherlands imposed an oil and steel embargo against Japan in 1941.

In the most successful cases _ such as sanctions in support of black majority rule in Africa or promoting democracy in communist Poland _ it took years to achieve the goal.

An ongoing U.S. trade embargo against Cuba, slapped on the island nation after Fidel Castro nationalized American holdings, was imposed six months before President Barack Obama celebrated his first birthday. A half century later, Castro and his brother Raul are still in power.

A 2007 study by three scholars at the Peter G. Peterson Institute for International Economics found that sanctions were successful in undermining or changing a regime in only about 34 percent of the cases. Other studies argue even that figure is too high because the Peterson study included "successful" cases where military force was also used.

Still, economic sanctions remain a popular option. They remain the "only coercive measures available to the international community" short of the use of force, as Jeremy Greenstock, a former British ambassador to the United Nations, wrote before the Iraq war.

The latest measures against Syria came Friday, when the Obama administration slapped new sanctions on the state-run oil company and its Lebanese ally Hezbollah, a Shiite militant group accused of helping prop up Assad.

They were largely symbolic: Americans have been banned from doing business with Hezbollah since the U.S. declared it a foreign terrorist organization in the 1990s, and longstanding U.S. sanctions against Syria have already blocked energy trade between the two countries. Obama blacklisted new imports last year.

But more robust U.S. and European Union sanctions have already dealt Syria a severe blow, even if they have yet to achieve their goals of ending the bloodshed.

U.S. and EU bans on oil imports which went into effect this year are estimated to be costing Syria about $400 million a month. Syria's foreign currency reserves are shrinking. Tourism, which accounted for more than 10 percent of Syria's gross domestic economy in 2010, has all but disappeared.

Still, some experts are skeptical that sanctions alone will force Assad to abandon what he considers a life and death struggle for power _ especially as long as he can count on support from Russia, China and Iran.

"The Assad regime does not take such steps any more seriously than previous sanctions," said Aram Nerguizian of the Center for Strategic and International Studies in Washington. "From the regime's standpoint, winning the battle against the insurgency and hanging on to political survival are paramount."

The Syrian experience points to the limits of the effectiveness of sanctions. Proponents acknowledge that to be most effective, sanctions must be applied by as great a number of countries as possible.

The U.N. Security Council has been unable to impose universally binding sanctions against Syria because its allies Russia and China can veto any proposal.

Even with universal sanctions, success in forcing change is by no means guaranteed.

Skillful autocrats _ Castro in Cuba, Robert Mugabe in Zimbabwe and Manuel Noriega in Panama to name a few _ have managed to shift the blame for their country's suffering on the foreign powers responsible for the sanctions.

Louis Kriesberg, a retired professor of social conflict at Syracuse University, wrote that economic sanctions "can widen the conflict, add to its destructiveness and sometimes prolong it."

Those arguments were reinforced by the experience of Syria's neighbor Iraq.

The Security Council imposed nearly total financial and trade sanctions on Iraq after Saddam Hussein invaded Kuwait in August 2000. After U.S.-led forces drove the Iraqis from Kuwait, the council reinforced the sanctions, linking them to Iraqi guarantees that they no longer held banned weapons of mass destruction.

Yet those sanctions remained in force for more than a dozen years as the government resisted U.N. efforts to verify that the banned weapons programs had been dismantled.

In the meantime, Saddam's cronies made fortunes smuggling oil and other commodities, while ordinary Iraqis suffered the effects of sanctions.

For years, visitors to Baghdad were shocked at the vast flea markets that sprang up all over the city. Women sold their jewelry, professors offered their personal libraries and retired soldiers hawked their medals _ all in a desperate effort to accumulate enough devalued cash to survive.

Some estimates placed the number of Iraqis who died during the sanctions era from lack of medicine or proper nutrition as high as 500,000, although the U.S. and Britain contested those figures.

"The lesson is that some countries cannot be influenced because the decision-makers do not attribute decisive weight to the potential damage done to their country," wrote Peter A.G. van Bergeijk, professor of international economics at the Dutch Institute of Social Studies.

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Associated Press writers Zeina Karam in Beirut and Bradley Klapper in Washington and AP researcher Monika Mathur in Washington contributed to this report.