In reality, far right groups in Ukraine are a small minority, and Ukraine has denied any involvement in the Russian Volunteer Corps or any role in fighting on the Russian side of the border. But Mr. Kapustin said that his group “definitely got a lot of encouragement” from the Ukrainian authorities. Some on the far right in Russia long ago soured on Mr. Putin, particularly for his jailing of so many nationalists, but also for his policies on immigration and for what they perceive as granting too much power to minorities like ethnic Chechens. Since the 2014 Maidan revolution and...