WASHINGTON -- Growing fear that war in Syria could unleash the world's first use of chemical weapons in nearly three decades is based on two grim scenarios -- neither considered likely but both carrying risks of civilian massacre and a major escalation of violence.
The first is that President Bashar Assad, in a last-ditch effort to save his regime, would order chemical attacks -- either as a limited demonstration to the rebels of his willingness to use the internationally banned weapons, or in a large-scale offensive designed to turn the tide of a conflict that already has killed tens of thousands.
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The second is that some portion of Assad's arsenal could be moved to Iran or Lebanon or fall into the hands of foreign fighters with ties to terrorist groups who are helping Syrian rebels.
What kinds of chemicals are in question? What weapons?
News this week that an unknown number of weapons in Syria were recently loaded with the nerve agent sarin brought the West's fears into sharp relief.
Syria has never confirmed that it even has chemical weapons. But it is believed to possess substantial stockpiles of mustard gas and a range of nerve agents, including sarin, a highly toxic substance that can suffocate its victims by paralyzing muscles around their lungs.
James Quinlivan, a Rand Corp. analyst who studies the elimination of weapons of mass destruction, said Syria is thought to have hundreds of tons of chemical weapons material, including not only sarin and mustard gas but possibly also the nerve agent VX, which, like sarin, kills by attacking the central nervous system.
Iraq's Saddam Hussein used sarin and mustard gas on Kurds in northern Iraq in a 1987-88 campaign that killed thousands. That was the last time state-controlled chemical weapons were used; a Japanese doomsday cult unleashed sarin in the Tokyo subway system in 1995, killing 13.
The precise dimensions of Syria's chemical weapons arsenal are not known, in part because it has never been subjected to outside inspection. Experts say it is a formidable collection, but the weapons date back almost 40 years -- when Assad's father, President Hafez Assad, began accumulating them -- and have not been modernized.
"Frankly, you'd stand as much chance of committing a self-inflicted wound as of actually killing opponents," said Aram Nerguizian, a Mideast security expert at the Center for Strategic and International Studies. "These systems are not going to achieve the end state that the regime wants, which is regime survival."
For example, the arsenal apparently does not include weapons that combine or mix chemical ingredients after a shell or missile is fired; instead the mixing must be done manually prior to launching the weapon, Nerguizian said.
U.S. officials have warned Assad there would be unspecified "consequences" if he used his chemical weapons or lost control of them. That could include military intervention, aided perhaps by allies such as Turkey. The U.S. and its allies might also launch a pre-emptive military operation to secure the weapons before they could be used.
One administration official, speaking on condition of anonymity because the official was not authorized to talk publicly, defined what would trigger a response: the use of chemical weapons, or movement with the intent to use them, or word that they were falling into the hands of a terrorist group like Hezbollah.
America and its allies have already begun preparing.
A U.S. special operations training team is in neighboring Jordan, teaching troops how to secure chemical stockpiles, according to one current and one former U.S. official briefed on the matter. The officials spoke on condition of anonymity because they were not authorized to discuss the issue publicly.
U.S. officials have said it could take as many as 75,000 ground troops to secure all of Syria's dozens of chemical sites in a worst-case scenario in which the intervention would face Syrian resistance. The Obama administration has been consulting with Turkey, Jordan, Israel, Russia and others on possible courses of action.
"We're prepared for the full range of contingencies," Pentagon press secretary George Little said Friday.
The U.S.-led NATO alliance this week agreed to move Patriot missiles to Turkey as a defensive measure. Patriots are capable of neutralizing a chemical warhead aboard a missile by incinerating it in flight, although a portion of the chemical could fall in populated areas.
Jeffrey White, a defense expert at the Washington Institute for Near East Policy, said that in addition to air-delivered weapons, the Syrian military can put chemical warheads on missiles like the Soviet-designed Scud, as well as artillery shells and short-range rockets and fire them into populated areas.
"Without intelligence warnings from external sources, rebel combatants and civilians would be highly vulnerable to surprise chemical attacks, increasing the chances for major casualties," White wrote in policy paper this week. In his view, Washington should be prepared for the "growing possibility" of chemical attacks in Syria.
Defense Secretary Leon Panetta said Thursday that the U.S. is concerned that "as the opposition advances, in particular on Damascus, that the regime might very well consider the use of chemical weapons."
Former CIA officer Reuel Marc Gerecht says Assad and his minority Alawite tribe view the fight against the rebels as a "war to the death." Thus, "it's not at all inconceivable that he would use" his chemical arms, he said.
"There is no telling what a mad dog will do when it's cornered," said Rep. Adam Schiff, D-Calif., a member of the House intelligence Committee. He added he still finds it hard to believe Assad would take that step.
Some experts believe a more likely scenario is that groups with terrorist ties who are helping the rebels might acquire some of Assad's weapons of mass destruction.
"I think the big problem is when and if Assad loses control of his weapons and sites," said David Friedman, a former head of the Israeli military's chemical-biological protection division. "Then, of course, weapons might fall into opposition hands and they might use it. This is a real danger and threat."
Quinlivan said Syrian Scud missiles carrying chemical warheads have a range of about 160 miles. That's just beyond the distance from the Syrian capital, Damascus, to Tel Aviv.
If Syria used chemical weapons, it would be violating international law -- specifically, the 1925 Geneva Protocol that bans the use of chemical and biological weapons. But because it is not a signatory to the 1997 Chemical Weapons Convention, Syria has reserved its right to produce and store chemical weapons. The only other countries that have not ratified that convention are Egypt, Israel, Angola, South Sudan, Somalia and Burma.
The chief of the organization in charge of implementing the Chemical Weapons Convention, Ahmet Uzumcu of Turkey, said Friday he wrote to Syrian authorities urging them to join the treaty as a way of assuring the world that Syria accepts that the use of such weapons is "completely contrary to global sentiment."
Earlier this week, Syria's deputy foreign minister, Faisal Mekdad, said in a TV interview that Syria would not use chemical weapons against its own people.
"We cannot possibly commit suicide," he said. "Syria is a responsible country."