This is an undeclared war that everyone must sign up for it in order for us to win.(Para.14)
Meaning: This is a war that is fought without any announcement, and everyone must take part in it if we wish to win the war.
sign up: sign an agreement to take part in sth.
I've signed up to take a course at the local college.我已报名在当地学院学一门课程。
He persuaded the company to sign up the singer.他说服公司与这位歌手签约。