U.N. chemical weapons inspectors surveyed the site of a possible attack Monday, while Secretary of State John F. Kerry strongly condemned Syria's alleged use of the weapons. The Washington Post's Ernesto Londoño discusses why chemical weapons elicit such strong reactions and have been so hard to eliminate. (The Washington Post)

The shelling of suburban Damascus with a suspected nerve agent last week was potentially the third large-scale use of a chemical weapon in the Middle East and may have broken the longest period in history without such an attack.

If confirmed, the attack, which U.S. officials say warrants a decisive military response from the West, would dash hopes that the world would never again see the large-scale use of chemical weapons, a prospect that had appeared increasingly realistic in recent years as all but a handful of nations signed a treaty agreeing to destroy their stockpiles.

If the weapons were deployed by the Syrian government, as Western officials allege, it would represent the first major chemical weapons attack by a nation against its own citizens since Saddam Hussein gassed Kurds in northern Iraq in 1988, an act so barbaric it galvanized the movement for a world free of chemical weapons.

The other two large-scale chemical weapons attacks in the region were carried out by Iraq against Iran during their war in the 1980s and by Egypt, which backed southern Yemen during the Yemen war in the 1960s.

Chemical weapons attacks have at times elicited strong and visceral reactions from the international community. Their possible occurrence in Syria is no exception, having drawn the United States and its allies closer than they have ever been to intervening militarily in a messy conflict in which the West has enemies on both sides of the front lines.

Although U.S. and other Western officials have alleged that Syria previously used chemical weapons on a smaller scale, the latest incident has prompted a more vigorous response.

“What we saw in Syria last week should shock the conscience of the world,” Secretary of State John F. Kerry said Monday at the State Department, vowing that the Syrian government would be held accountable for what he called a “moral obscenity.”

Kerry said he had been repulsed by footage of “bodies contorting in spasms” and a searing image of a father holding up his child’s corpse, calling them pictures of “human suffering we can never ignore or forget.”

While tens of thousands of Syrians have been killed in shelling, bombings, ambushes and shootings, no other act in the country’s conflict, which has been raging since 2011, has drawn such strong condemnation from the United States. Experts say the alleged use of nerve agents is particularly jarring because such attacks are indiscriminate, lead to blood-curdling footage of the aftermath and have long-lasting consequences for survivors.

“There is something kind of grotesque about these weapons,” said Jeffrey Lewis, an expert on weapons of mass destruction at the James Martin Center for Nonproliferation Studies. “It’s a pretty awful way to die, and there are long-term effects for victims, who are left with ugly health-care problems.”

World War I was the first conflict that featured widespread use of chemical weapons. At least 90,000 people were killed and hundreds of thousands more were wounded with those munitions during the war, according to the Organization for the Prohibition of Chemical Weapons, which monitors global disarmament efforts.

During the Cold War, the United States and the Soviet Union raced to build enormous chemical weapons stockpiles. In 1969, President Richard Nixon, alarmed by the cost of America’s chemical and biological weapons, announced that the United States would shut down its offensive programs for both types of weapons, saying that their use was “repugnant to the conscience of mankind.”

Both countries embarked on a costly and protracted effort to destroy their stockpiles. The undertaking gained steam as the United States ratified the Chemical Weapons Convention treaty in 1997 and vowed to get rid of its roughly 30,500 tons of munitions by 2012. Washington missed the deadline, but the Army remains committed to destroying the chemical weapons still securely stored at a handful of sites.

The only countries that have not signed the treaty are Angola, Egypt, North Korea, South Sudan and Syria. Israel has signed but not ratified it.

Experts think Syria has one of the world’s largest stockpiles of chemical weapons, including sarin, tabun, mustard gas and the nerve agent VX.

Several Middle Eastern nations stocked up on chemical weapons over the past century, along with the rest of the world, but few deployed them. Egypt did so in the 1960s during the civil war in North Yemen, using Soviet-supplied munitions. But Western nations, which were eager to build partnerships with Egypt at the time, largely ignored evidence of those attacks.

Saddam Hussein, the former Iraqi dictator, used nerve agents during his country’s war with Iran in the 1980s. Washington, which feared an Iranian victory, was well aware of Iraq’s use of chemical weapons but nonetheless supplied Baghdad with intelligence, according to declassified intelligence reports disclosed Monday by Foreign Policy.

As that conflict was coming to a close, Hussein ordered a massive gas attack on Halabja, a Kurdish village in northern Iraq, killing thousands. The operation, launched to quell a Kurdish uprising, was the largest chemical attack in history and was one reason Iraq became a global pariah in the following decades.

U.S. officials have said military action in response to last week’s attack in Syria is imminent, but they have not said exactly what they are contemplating. The Obama administration has been extraordinarily reluctant to inject itself militarily into the conflict, in large part because it is not confident that doing so would influence the war in a way that would advance U.S. interests.

Iran and the Shiite Lebanese militant group Hezbollah are backing the Syrian government, while Sunni insurgents affiliated with al-Qaeda are among the most effective fighters on the rebel side.

“This is why you haven’t seen anything happen,” said Cindy Vestergaard, a senior researcher who specializes in chemical weapons at the Danish Institute for International Studies. “There is a big potential for a proxy war with Iran.”