YOU SAID:
they understood the assignment
INTO JAPANESE
彼らはその任務を理解した
BACK INTO ENGLISH
they understood the mission
INTO JAPANESE
彼らは使命を理解した
BACK INTO ENGLISH
they understood their mission
INTO JAPANESE
彼らは彼らの使命を理解した
BACK INTO ENGLISH
they understood their mission
That didn't even make that much sense in English.