On Monday, the United States announced that it would be joining the growing list of vexed nations expelling Russian diplomats and officials from their borders, following the use of a Soviet-era nerve agent on a former Russian spy on British soil.

With 60 Russians being ordered out of the United States and at least 40 more from 18 European nations and Canada, the exodus of Kremlin officials headed back to Moscow is perhaps not unprecedented, but historic nonetheless, and on Tuesday, American Secretary of Defense James Mattis did not mince words regarding his stance on the topic.

On March 4th, a Soviet produced nerve agent known as Novichok is believed to have been used in the attempted assassination of former Russian Military Intelligence officer turned MI6 informant, Sergei Skripal. Exposure to the weapon left both Skripal and his 33 year old daughter in critical condition, along with the first responding officer in the shopping square of Salisbury, England. At least 30 others ultimately had to seek treatment following exposure to the substance that the international community has claimed came from Soviet labs. Russia, however, has countered with a number of contradicting claims, all placing the blame for the incident on someone else.

According to Mattis, there’s little question as to who was responsible for the attack. When it came up during a press briefing on Tuesday he referred to the incident as, “the Russian … apparent, pretty obvious, Russian use of a weapon of mass destruction, a chemical agent, for the first time in Europe since World War II.”