Why does Russia think there are Nazis in Ukraine?

In 2014, Russian separatists took over government buildings in the Donbas region of eastern Ukraine. Russia took advantage of this chaos to pour weapons and volunteers into the region.

Ukrainian militias formed to fight the separatists. One of these units was a battalion of white supremacists. Ukraine's constitution guarantees freedom of speech, so this militia battalion was unchanged when it was integrated into the Ukrainian Army.

Russia used the existence of this battalion as proof that Ukraine was run by Nazis for a pretext to invade.