Islamic State extremists released a video Tuesday purportedly showing the beheading of a second American journalist, Steven Sotloff, and warning President Barack Obama that as long as U.S. airstrikes against the militant group continue, “our knife will continue to strike the necks of your people.” The footage — depicting what the U.S. said appeared to be a sickening act of brutality — was posted two weeks after the release of video showing the killing of James Foley and days after Sotloff’s mother pleaded for his life. Barak Barfi, a spokesman for the Sotloff family, said that the family had seen...