On FB, when you reach war level 3, you participate in the war game.
If you refrain from war, and stay in lower war levels, you cannot be attacked for 60 days.
after 60 days you are moved automatically to war level 3 and you are part of the war game.
This is explained several times in messages you get when you join and warnings that when on FB, you must use the 60 days to prepare for war and refrain from war before and not move to war level 3 before the 60 days are over.
Please also read the beginners document. It is explained there too.