When I saw the first images of armed men in ragtag uniforms taking over the Russian city of Rostov-on-Don on Saturday morning, I was immediately reminded of the “little green men” who began showing up in cities in Crimea and eastern Ukraine in 2014. Like the Russian troops and soldiers-of-fortune who began the Kremlin’s covert invasion of Ukraine, the fighters in Rostov displayed no insignia as they seized key buildings, including the headquarters of Russia’s Southern Military District. Just as…