Hours after the Syrian regime suffered its greatest setback in the yearlong civil war, the U.S. and U.K. defense chiefs feared that dictator Bashar Assad might use his stockpile of chemical weapons – the results of what may be the largest active chemical program on the planet. But because of the structure of Assad's extensive chemical weapons effort, stopping him from using his weapons may not be possible, even if the U.S. military suddenly decided to openly intervene.

U.S. Defense Secretary Leon Panetta said Assad's rule was "rapidly spinning out of control" after a suicide bomber in Damascus killed the defense minister and possibly other high-ranking security officials. But that raises questions about whether a desperate Assad would turn to his large stockpiles of sarin, VX and mustard gas in order to cling to power. Reportedly, Assad has begun moving his chemical weapons out of storage.

In a joint Pentagon press conference with Panetta on Wednesday morning, Philip Hammond, the U.K. defense chief said that all nations "share a necessity to see these weapons kept under tight control and not used in any way, shape or form." Panetta said the U.S. would hold the Assad regime "responsible should anything happen with regards to those [chemical weapons] sites."

But when asked whether the United States and United Kingdom could "stop [Assad] from using chemical weapons," Hammond merely said, "the international community can apply pressure to ensure that the regime understands that we will not tolerate the use or the proliferation of these chemical weapons."

Hammond's non-answer says a lot. Preventing the use of chemical weapons, or the proliferation of them in any post-Assad chaos, is a wicked challenge.

And there is little appetite in the U.S. military for taking on that challenge. Globalsecurity.org estimates Syria possesses "hundreds of liters" of Sarin, mustard gas and VX. The Pentagon estimates that 75,000 troops would be required to secure the chemical stockpiles, a force nearly the size of the one the U.S. fields in Afghanistan.

In part, that's because Syria's chemical weapons are stored in hardened targets that would necessitate using giant bunker-buster bombs to destroy. Taking the weapons out before they make it to storage isn't easy: They're stored all over Syria, so destroying the facilities will require a major bombing campaign – and Assad has sophisticated air defenses, maybe including Russia's powerful S-300 missiles. And that's assuming the United States knows where they are: Globalsecurity.org calls the manufacturing sites "notoriously small" and "difficult to conclusively identify."

Nor does the danger stop when the weapons are on the move. Assad is believed to store his chemical weapons in binary form, meaning the chemical precursors for the weapon are kept in separate containers and need to be mixed together before they're placed in a single warhead. That makes them easier to transport, because there's no risk of volatile chemical weapons injuring or killing their chauffeurs.

All this is a result of decades' worth of development. In the mid-1990s, the infamous Russian general and chemical weapons expert Anatoly Kuntsevich helped the regime upgrade its chemical capabilities by smuggling precursor materials for the deadly VX nerve agent into Syria (.pdf) – an act for which his patron, President Boris Yeltsin, fired him. Acquisition of precursor chemicals continues, U.S. intelligence believes, under cover of Syria's pharmaceutical and cosmetics industry.

Stopping the precursor chemicals from entering Syria is difficult. The U.S. supposedly has a big effort to block the chemicals, and special equipment for processing them, before they enter Syria. Shipping companies sometimes call off deals; at the other end of the spectrum, boxes sometimes mysteriously go missing from shipyard stocks. But even with that effort, the estimates for precursor chemicals – like sodium sulfide and the yellow phosphorus necessary to make nerve gas– arriving in Syria over the past several years alone run in the tens of tons.

The program faces technical challenges. Syria has to brew its deadly chemicals in small doses, owing to a lack of specialized industrial equipment. Still, the fact that Syria has yielded so many chemical weapons despite that lack is an indication of how intensely the regime values the illicit weapons.

Wednesday's bombings may have shown Assad that, as Panetta said, his end is visible. But if he uses his chemical weapons, or spirits them away in an act of revenge, there may not be much Panetta can do about it.