Although the death toll in Syria from conventional weapons has surpassed 100,000, it wasn't until the U.S. said it determined that Assad's government had used chemical weapons on a large scale that President Barack Obama seriously threatened military action.
That's because chemical weapons, Obama says, are different.
"Men, women, children lying in rows, killed by poison gas. Others foaming at the mouth, gasping for breath," Obama told the nation Tuesday night, describing the Aug. 21 attack that the U.S. says killed more than 1,400 Syrians, including hundreds of children.
Left unsaid was that the remaining U.S. stockpile includes many of the same chemicals in Assad's possession. The Syrian regime has more than 1,000 tons of sulfur, mustard gas and the ingredients for sarin and the nerve agent VX, Kerry told Congress this week.
Under a tenuous diplomatic deal being coordinated by Russia, which holds the world's largest remaining chemical weapons stockpile, Syria would join the Chemical Weapons Convention, declare its stockpiles and hand them over to the international community for destruction, all to avert a punitive U.S. military strike.
It's unclear how that task can be carried out when there's distrust of Syria in the international community, uncertainty about the weapons' locations and ongoing fighting between Assad's forces and rebels. The White House says it will require extensive verification to ensure that stall tactics aren't disguised as legitimate delays.
In the U.S., those delays have ranged from environmental ones and political opposition to technical and safety challenges to tough laws restricting the transport of chemical weapons. It's also been difficult to round up the tens of billions of dollars to pay for destroying the cache.
"All of this is a slow process," said Dieter Rothbacher, a former U.N. chemical weapons inspector who has worked in Iraq, Russia and the U.S. "Falling behind (schedule) is actually relatively easy."
The U.S. started developing chemical weapons around World War I, steadily increasing its capabilities through World War II until 1968. The stockpile grew to about 31,500 tons of sarin, VX, mustard gas and other agents, according to the Army. Russia, by comparison, has said it amassed about 44,000 tons.
The move toward destroying the United States' chemical weapons started in the 1970s, building momentum in the 1980s when Congress directed the Defense Department to start eliminating the stockpile.
That commitment became an international obligation when the U.S. signed the Chemical Weapons Convention in 1993 and ratified it four years later. That started the clock on a 10-year period in which the U.S. was supposed to destroy the rest of its chemical weapons.
The process is complex.
The two basic methods chemical neutralization and incineration both require specialized facilities. Using incineration, chemicals must be heated to thousands of degrees. Decades-old storage containers can be leaky and tough to handle. Destruction produces highly hazardous waste that must be carefully stored. And assembled weapons, like those chemicals already loaded into rockets and packed with explosives, pose their own dangers.
The Army used to destroy chemical weapons at nine sites across the U.S. By January 2012, troops had completed 90 percent of the job, and only two active sites now remain.
The U.S. has long since missed its original 2007 deadline, which was extended to 2012, then missed again. Russia is behind schedule too.
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Associated Press writer Josh Lederman in Washington and AP Science Writer Seth Borenstein contributed to this report.
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Follow Josh Lederman on Twitter at http://twitter.com/joshledermanAP
