The cable was included in a trove of State Department messages leaked to WikiLeaks in 2010.

Another leaked State Department cable on the Syrians asserted that “part of their modus operandi is to hide procurement under the guise of legitimate pharmaceutical or other transactions.”

Publicly, American officials contend that they have done much since then to limit the flow of raw materials that feed Syria’s chemical weapons industry, in particular Syria’s Scientific Studies and Research Center, which has been identified as a principal government enterprise for weapons development. Israel struck a missile convoy outside the center in January, American intelligence officials have said, on suspicions that weapons were headed for delivery to Hezbollah in Lebanon.

“For several years, the Treasury Department, working with our partners across the U.S. government, has taken steps to expose and disrupt the Syrian regime’s W.M.D. proliferation activities,” David S. Cohen, the Treasury under secretary in charge of sanctions, said in an e-mailed statement. “We will continue to use all of our authorities to undermine the Syrian government’s W.M.D. proliferation efforts within Syria as well as around the world.”

The diplomatic cables and other intelligence documents show that, over time, the two generations of Assads built up a huge stockpile by creating companies with the appearance of legitimacy, importing chemicals that had many legitimate uses and capitalizing on the chaos that followed the collapse of the Soviet Union. A Russian general responsible for dismantling old Soviet chemical weapons, who died a decade ago, was identified by a colleague as the man who helped the Syrian government establish its chemical weapons program.

As early as 1991, under the first Bush presidency, a now declassified National Intelligence Estimate concluded that “both Czechoslovakia and the Soviet Union provided the chemical agents, delivery systems and training that flowed to Syria.” The same report concluded that Syria most likely possessed 500-kilogram aerial bombs containing sarin — larger, it appears, than the warheads mounted atop rockets that killed so many in the Ghouta suburbs of Damascus on Aug. 21.

While investigations by the United Nations into the attack remain incomplete, a group of outside academic experts reported last week that some of the warheads used contained 100 kilograms or more of sarin agent.

But even with such a large stockpile of weapons on hand, the director of national intelligence, James R. Clapper Jr., reported to Congress earlier this year that Syria “remains dependent on foreign sources for key elements” of its program. That dependence points to an important vulnerability that the West may be able to exploit as it tries to stop Syria from expanding its program.