这个which后面有没有逗号?the primary role of government which is to protect its citizens

这个which后面有没有逗号?
the primary role of government which is to protect its citizens

为什么要加which呢?
没有which是个完整的句子,加了which看不懂,后面还有句子?
the primary role of government is to protect its citizens.

不用加逗号，which后面是引导从句，不需要加逗号。 无论前面有没有逗号，这句话都有点指代不明确。 这个which指代的是什么？是something，还是when