You should have been taught that an attack can come from any direction (even below, by having your ankles grabbed).

There are several things you can do if assaulted from behind, depending upon the circumstances.

For instance, if grabbed from behind (arms over your arms) you can thrust your buttocks back to gain some room while stomping -- HARD! -- on the instep. Then turn and assault the eyes by ramming the butt of your palm into the chin (HARD!) while clutching at the eyes (slide up the face to get under glasses).

If you are put into a choke hold, quickly turn your face right or left -- this will help stop your throat from being crushed or your air cut off. This for an "arm hold," not a hand choking and works from front or back. Turn into the crook of the assailant's elbow.

Never fight a knife, gun, broken bottle, etc. unless you are a) absolutely certain there is no other choice and that the chance of death or injury exceeds the certainty, and b) you are reasonably certain that you can hurt the assailant before the assailant hurts you -- and that the damage you do will be greater than whatever he can do and, c) you know what you're doing. Luck is fine, but it runs good AND bad.

You are a weapon. Use it when you have to.