From the Document: "On August 6, 2018, Secretary of State Michael Pompeo determined that in March 2018 the government of Russia used a chemical weapon in the United Kingdom in contravention of international law. This finding triggered requirements under the Chemical and Biological Weapons Control and Warfare Elimination Act of 1991 (CBW Act; title III, P.L. 102-182; 22 U.S.C. 5601 et seq.). The CBW Act requires the President to impose economic and diplomatic measures that first cut off foreign aid, arms sales, and export licenses for controlled goods, services, and technology, and ratchet up in intensity if certain conditions are not met within 90 days. As of August 2019, the United States has imposed two rounds of sanctions on Russia as required by the CBW Act."
CRS In Focus, IF10962
Congressional Research Service: https://crsreports.congress.gov/