There's a lot of them, and no way for the West to get control of them

Stanley Kurtz at NRO's The Corner says that Syria's collection of chemical weapons is huge and has been shifted around form particular locations to others over the past year-plus.

As the Assad regime was losing its grip on power in early 2012, American officials began to warn Iraq, Jordan, Lebanon, and Saudi Arabia that chemical weapons might be crossing their borders, and offered to help with the problem. Meanwhile, Israeli officials registered concern about potential chemical weapons transfers from Syria to Hezbollah.

Louder alarms went off mid-July of 2012, after intelligence indicated that the Assad regime was removing chemical weapons from storage. It was impossible to tell at the time whether this was being done to secure those weapons against rebel advances, as preparation for use, or to pass some on to Syria’s terrorist allies. In any case, more than a month before President Obama drew his red line, lower-level American officials began to do so publicly. Addressing the Assad regime, for example, Defense Department spokesman George Little said, “We would caution them strongly against any intention to use those weapons. That would cross a serious red line.”

Notice Little’s focus on internal use, rather than proliferation. At the time, however, a senior Syrian defector said that a desperate Assad would not hesitate to use chemical weapons to save himself, while unconfirmed reports indicated that chemical weapons may already have been used against the rebels. It should have been clear at that point that drawing a red line against use, rather than proliferation, could very well result in American entry into Syria’s civil war.

Within a week of the revelation that Syria was taking chemical weapons out of storage, high American officials were in discussions with an increasingly alarmed Israel, now considering its own raid to destroy Syria’s chemical weapons facilities.

Also in late July of 2012, former American Ambassador to Israel and Brookings Institution Foreign Policy Director, Martin Indyk advocated for a strong red line and (mistakenly) predicted that were Assad to use chemical weapons, Russia would likely drop its opposition to tough UN sanctions. At the same moment, Syria expert Andrew Tabler called on the American government to draw a very public red line against chemical weapons use in Syria, while standing prepared to lead an independent military coalition against Syria should Russia obstruct punishment at the United Nations.
Responding to Tabler, foreign policy commentator Daniel Drezner argued that no Western threat, however credible, would prevent Assad from using chemical weapons to save his own skin. “Let’s be blunt,” added Drezner, the only military response that would matter would be a “full-blown ground invasion.” Military planners estimate that it would take 75,000 ground troops to secure Syria’s massive chemical weapons storehouse. And even with 200,000 American and coalition troops in Iraq, many sophisticated explosives were looted after Saddam fell. So Drezner was effectively arguing that Tabler and the strong-red-line camp were bound to pull us into either an invasion or a humiliating climb-down.

Nonetheless, on August 1, 2012, less than two weeks after Indyk and Tabler called for a strong public red-line against chemical weapons use in Syria, both men were featured witnesses at a Senate Foreign Relations Committee Hearing chaired by then-Senator John Kerry.

There was some confusion at this hearing about the actual nature and status of the American red line on chemical weapons. Without revealing classified details, Kerry emphasized that red lines of some sort had already been privately conveyed to key parties in Syria. 

Because of this glaring lack of certainty as to the whereabouts and security of the weapons, Kurtz says that the red line should have been against proliferation rather than internal use.  That the MEC went the opposite route has led to yet another no-good-options scenario.