In 2014, an affiliate of the Islamic State of Iraq and Syria emerged in Afghanistan. Wilayat Khorasan, or ISIS-K, intends to secure Afghanistan to legitimize the Islamic State’s caliphate across the ‘Khorasan Province’ including portions of Central Asia, China, Iran, the Indian Subcontinent, and Southeast Asia. While the group’s intent is clear, its capability confounds analysts. The authors argue that Wilayat Khorasan is likely the Islamic State’s most viable and lethal regional affiliate based on an expansionist military strategy. This is designed to enable the group’s encirclement of Jalalabad City in Nangarhar Province and is foundational to its expanded operational reach, regionalization, and lethality. Since 2016, the US-led Coalition’s counter-terrorism strategy has disrupted ISIS-K’s critical requirements and prevented external attacks. Yet, raids and strikes alone will not defeat ISIS-K. They must be calibrated against an institution-building approach that legitimizes Afghanistan’s government and redresses grievances that ISIS-K exploits to resolve.