ARMS CONTROL is a method to lock up dangerous weapons, but it has always depended on the political will of states to comply with their commitments. When the Cold War superpowers found it in their interest to reduce nuclear weapons in a way that was verifiable, they did; when they did not, the arms race zoomed ahead. The use of a nerve agent to kill civilians in Syria recently is another reminder. Dictator Bashar al-Assad and his regime relinquished a large stockpile of chemical weapons under pressure but concealed some, or made new stocks, to eventually kill again, possibly with the connivance of Russia and Iran. The arms-control agreement Damascus signed did not protect the civilians who were murdered.

After the Ghouta chemical weapons attack in August 2013, which killed more than 1,400 people, Mr. Assad agreed that Syria would join the Chemical Weapons Convention, which prohibits the “development, production, acquisition, stockpiling, retention, transfer or use of chemical weapons.” After an agreement between the United States and Russia, some 1,300 tons of Mr. Assad’s declared chemical weapons stocks were removed and destroyed at sea, and the declared Syrian production facilities were destroyed. This was a worthy endeavor; one can only imagine the consequences if the Islamic State had seized the chemical arsenal.

But the key word is “declared.” Ever since the removal operation, suspicions had been growing that Mr. Assad possessed an undeclared cache. Colum Lynch and David Kenner of Foreign Policy revealed in August the contents of a 75-page report by the Organization for the Prohibition of Chemical Weapons (OPCW), which described a troubling pattern of coverup and obfuscation. In a confidential summary of the report, OPCW Director General Ahmet Uzumcu said most of the 122 samples taken at “multiple locations” in Syria indicated “potentially undeclared chemical weapons-related activities.” Only the month before, Mr. Lynch and Mr. Kenner reported, the U.S. representative to the organization, Kenneth Ward, said Syria had engaged “in a calculated campaign of intransigence and obfuscation, of deception, and of defiance.” Syria also repeatedly used chlorine as a chemical warfare agent.

The Obama administration oversold the Syrian destruction operation as 100 percent effective. That was a mistake. The truth about these weapons is that even when small amounts slip through, they can do terrible damage. The 1995 Aum Shinrikyo subway attack in Tokyo, which killed 12 people, injured more than 1,000 and set off mass panic, involved the release of 159 ounces of sarin.

In recent years, there has been far too much complacency about the dangers of non-nuclear weapons of mass destruction, including the recent failure to strengthen the Biological and Toxin Weapons Convention. Chemical and biological substances that cause great harm can be easily concealed. Arms-control agreements work when they are verifiable, with intrusive inspections. But they can be subverted. Mr. Assad showed his true colors by cheating and killing. Anyone thinking about an arms-control deal with Kim Jong Un over North Korea’s expanding missile and nuclear programs should keep this lesson in mind.