As U.S. forces withdraw from Afghanistan, the battlefield they leave behind is changing dramatically and becoming more deadly.

No longer pinned down by U.S. air cover, Taliban fighters are attacking Afghan military posts in larger numbers with the aim of taking and holding ground — a shift from the hit-and-run strikes with posses of gunmen, explosives and suicide bombers.

Struggling to hold the insurgents back, the riposte from commanders of the Afghan security forces has been clear: across the country, they are now telling their men to kill captured Taliban fighters instead of taking prisoners.