The Bolsheviks fought the forces of the pro-Romanov monarchists, reformist Social Democrats, former Imperial Army officers and soldiers in the anti-communist White Armies along with several foreign nations sending in interventionist forces, aid and supplies for the White Armies. Despite this, Vladimir Lenin regarded the left-wing opposition as the most threatening the Bolshevik regime faced, describing the Kronstadt rebellion as "undoubtedly more dangerous than Denikin, Yudenich, and Kolchak combined".