An appalling video emerged Tuesday showing what appears to be a Russian soldier savagely beheading a Ukrainian prisoner of war with a knife, leading Kyiv to compare Vladimir Putin’s regime to ISIS. The footage, which has been circulating on pro-Russian Telegram channels, shows an apparent Russian serviceman slicing into the neck of a captive wearing a yellow armband associated with Ukrainian forces. The unidentified victim is still alive at the start of the decapitation and is heard begging: “It hurts, stop.” The executioner’s comrades cheer him on in the background, urging him to “f—ing cut it, break the spine! What,...