THE PRINCIPAL achievement the Obama administration might claim in an otherwise tragically failed response to Syria’s civil war is eroding. Last September President Obama brokered an agreement with Russia under which the regime of Bashar al-Assad was to give up its stockpile of chemical weapons and join the Chemical Weapons Convention, which prohibits production or use of those horrific arms.

Yet months after the expiration of the February deadline for removing all chemical stocks from Syria’s territory, the regime not only retains a substantial stockpile but also has returned to assaulting civilian areas with chemicals. The Obama administration’s response is all too familiar: It is trying to avoid acknowledging those facts.

Administration spokesmen boast that 92.5 percent of Syria’s chemical weapons and precursors have been removed from the country for destruction by the end of June. But Damascus is dragging its feet on delivering the last 27 tons of chemicals used to make deadly sarin gas. According to The Post’s Ernesto Londoño and Greg Miller, U.S. officials believe the Assad regime is using the stocks as leverage to retain a network of tunnels and buildings that could be used as storage or production facilities, which the Organisation for Prohibition of Chemical Weaponswants destroyed.

Meanwhile, British, French and U.S. intelligence analysts have concluded that Syria is probably hiding part of its arsenal that it failed to declare, including stocks of sarin and mustard gas, according to news reports. State Department spokeswoman Jen Psaki confirmed last week that the United States has been skeptical about whether Assad has revealed the extent of Syria’s chemical weapons stockpile.

Finally, evidence is piling up that Assad’s forces have been dropping bombs filled with chlorine on opposition-held areas. France’s foreign minister told reporters in Washington on Tuesday that there had been at least 14 such attacks since October. Laurent Fabius, who said “things would have been different” had Mr. Obama not backed away from using force in response to a chemical weapons attack last August, said the “regime is still capable of producing chemical weapons and is determined to use them.”

Ms. Psaki said April 21 that the United States had “indications” of the use of chlorine, which is not one of the chemicals Syria was obliged to surrender. But the Obama administration has taken the position that it must await an investigation by the OPCW before reaching a definite conclusion. Meanwhile, the chlorine attacks have continued. An unnamed senior U.S. official offered Mr. Londoño and Mr. Miller a frank explanation of this filibuster: “There’s reluctance to call attention to it because there’s not much we can do about it.”

There are, of course, many actions Mr. Obama could take to punish Syria for its use of chemical weapons and to prevent their further deployment. He could begin by granting the opposition’s request for antiaircraft missiles to use against the helicopters that are dropping chlorine bombs. He could revive his plan to launch U.S. military strikes against Syrian infrastructure that supports those attacks.

In reality, Mr. Assad is being allowed to disregard his chemical weapons commitment with impunity not because there’s nothing the United States can do but because Mr. Obama chooses to do nothing.