Bangladesh, Aug. 21 -- The alleged plot to bomb the New York State Capitol is a disturbing reminder that the threat posed by jihadist extremism in the United States has not disappeared. It has changed form. Rather than relying exclusively on large, centrally directed terrorist networks, contemporary extremist movements can inspire individuals to radicalize themselves, identify targets, acquire weapons or bomb-making materials, and act with little or no direct contact with an overseas organization.

According to federal prosecutors, Samantha Bowie allegedly pledged allegiance to ISIS and developed a plan to attack the New York State Capitol before fleeing the United States to join the terrorist organization. Investigators say she expressed...