Zhànshí tōngdí shì zuì yánzhòng de zuìxíng zhī yī.
Collaborating with the enemy during wartime is one of the most serious crimes.
Tips
usage
通敌 often appears with 叛国 (to betray one's country) as the compound 通敌叛国, emphasizing both the act of contact with the enemy and the treason it constitutes.