Pakistan, Sept. 29 -- When the Taliban marched back into Kabul some half-decade ago, assurances were made to the world and most directly to neighbouring nations that Afghan territory would never again become an engine for regional instability. Yet, the reality unfolding across the border tells a vastly different story. Behind a wall of official denials, a deeply entrenched militant nexus has taken root. It is turning eastern Afghanistan into a well-fortified sanctuary, a sophisticated training ground and a staging platform for relentless cross-border violence targeting its neighbour, Pakistan.

The discrepancy between rhetoric and reality is laid bare on the ground. Detailed dossiers have identified hundreds of Afghan nationals killed whi...