In April 2018, Lee was one of eight Republican senators to sign a letter to United States Secretary of the Treasury Steve Mnuchin and acting Secretary of State John Sullivan expressing "deep concern" over a United Nations report exposing "North Korean sanctions evasion involving Russia and China" and asserting that the findings "demonstrate an elaborate and alarming military-venture between rogue, tyrannical states to avoid United States and international sanctions and inflict terror and death upon thousands of innocent people" while calling it "imperative that the United States provides a swift and appropriate response to the continued use of chemical weapons used by President Assad and his forces, and works to address the shortcomings in sanctions enforcement." He criticized Trump for ordering the 2018 missile strikes against Syria in response to the Douma chemical attack, stating that he lacked the constitutional authority to do so without Congress's permission because the U.S. was not in imminent danger. Lee supported Trump's decision to withdraw American troops from Syria in December 2018, saying that American forces should not have been in the country anyway without Congressional authorization. He said that the Obama administration had not made clear American objectives in Syria surrounding Assad's future, and that he believed Trump's claim that the Islamic State had been defeated.