“It was frustrating,” Juan C. Zarate, a former deputy national security adviser for combating terrorism in the George W. Bush administration, recalled Friday.

“People tried. There were always other understandably urgent priorities — Iran’s nuclear program, North Korea,” said Mr. Zarate, who has written a book about American efforts to crack down on illegal financing for terrorist groups and states including Syria, Iran and North Korea. “It was an issue that was always there, but never rose to the top of the world’s agenda.”

Proliferation experts said President Bashar al-Assad of Syria and his father before him, former President Hafez al-Assad, were greatly helped in their chemical weapons ambitions by a basic underlying fact: often innocuous, legally exportable materials are also the precursors to manufacturing deadly chemical weapons.