The revelation that a senior Pakistani army officer was detained on suspicion of ties to a radical Islamist group has raised fresh concerns about the reach and influence of an organization that has long vexed analysts and politicians. Hizb-ut-Tahrir says it rejects violence but uses venomous rhetoric and pushes for military coups. It seeks the overthrow of Pakistan's elected government and wants to unite the Muslim world under one government following strict Islamic law. Pakistan's army...