Afghanistan, Dec. 19 -- Renewed violence along the Afghanistan-Pakistan border in recent months has underscored not only the fragility of relations between Kabul and Islamabad, but also the constraints facing China's role as a regional diplomatic actor. Exchanges of fire near major crossings, Pakistan's airstrikes that resulted in civilian casualties inside Afghanistan, Kabul's retaliatory responses, and repeated border closures mark a departure from the sporadic incidents of previous years. What has emerged instead is a more entrenched cycle of escalation, with both sides attempting to assert red lines along the disputed Durand Line.
At the core of this confrontation lies a structural impasse that has resisted external mediation. Pakistan...