So what prompted this latest, and arguably more severe, retaliation? Technically, U.S. law. Under the Chemical and Biological Weapons Control and Warfare Elimination Act of 1991, the United States is required to impose sanctions on any foreign power determined to have used chemical or biological weapons in violation of international law within 60 days of assigning blame to the country in question. As the Republican lawmaker Ed Royce pointed out in a letter to President Trump last month, the administration missed that deadline by more than a month.

Russia, which has long denied involvement in the Novichok attack, dismissed the new U.S. sanctions as “draconian” and a threat to the “constructive atmosphere” of the summit between Trump and Russian President Vladimir Putin last month in Helsinki, during which Trump appeared to accept Russia’s denials that it interfered in the 2016 U.S. presidential election. Not long after that, Trump invited Putin to the White House.

“The theater of absurd continues,” Dmitry Polyanskiy, the first deputy permanent representative of Russia to the United Nations, said Wednesday in a tweet. “No proofs, no clues, no logic, no presumption of innocence, just highly-liklies. Only one rule: blame everything on Russia, no matter how absurd and fake it is. Let us welcome the United Sanctions of America!”

These sanctions likely won’t be the last. If Russia does not provide assurances that it will no longer use such weapons within the next three months, the U.S. government has warned that it will move to impose even “more draconian” sanctions, including a wider trade embargo and a possible downgrade in diplomatic relations between the two countries. But Moscow is unlikely to be swayed by threats of additional penalties. “Russian denials will continue—there is no doubt about that,” Duncan Allen, a Russia and Eurasia specialist at the London-based Chatham House, told me. “More U.S. sanctions frankly are only a matter of time.”