A major victory in the fight against extremism was secured last month when YouTube announced that it had taken down the lectures and sermons of Anwar al-Awlaki, an American who had turned into a firebrand preacher and notoriously effective recruiter for Al Qaeda in the Arabian Peninsula. In 2011 President Barack Obama took the extraordinary step of ordering a drone strike on Mr. Awlaki in Yemen, citing his “lead in planning and directing efforts to murder innocent Americans.” Unfortunately, given the ubiquity of his lectures and sermons online, Mr. Awlaki continued to inspire and incite extremist violence after his death. YouTube’s acknowledgment of the harmful impact of the Awlaki sermons should also open the door to a broader and more consistent approach against the content posted by the Islamic State online. The group’s sophisticated exploitation of the internet to spread its message must be matched by a systematic and resource-intensive effort that includes website hosting companies such as WordPress, encrypted communications platforms such as Telegram and file-sharing platforms such as JustPaste.it, apart from video platforms such as YouTube.