Use of one of the emblems in order to protect combatants and military equipment with the intent of misleading an adversary, perfidy, is considered a war crime.
Impersonating soldiers of the other side by wearing the enemys uniform is allowed, though fighting in that uniform is unlawful perfidy, as is the taking of hostages.
It admits of deception, but disclaims acts of perfidy; and, in general, military necessity does not include any act of hostility which makes the return to peace unnecessarily difficult.