Perfidy

"In the context of war, perfidy is a form of deception in which one side promises to act in good faith (such as by raising a flag of truce) with the intention of breaking that promise once the unsuspecting enemy is exposed (such as by coming out of cover to attack the enemy coming to take the "surrendering" prisoners into custody). Perfidy constitutes a breach of the laws of war and so is a war crime, as it degrades the protections and mutual restraints developed in the interest of all parties, combatants and civilians."

https://en.wikipedia.org/wiki/Perfidy