5.56 does not equal 223?

I know that it is recommended that you don't shoot NATO 5.56 in a civilian 223 Remington. I first thought they were the same but apparently there's a difference that makes it dangerous. Does someone know what the difference is and why they are made different?
Dean

pressure...

Yup, pressure is the biggie. There are other reasons, too. Break actions especially can not handle really hot mil loads. Many non-AR .223's have barrels with twists that won't stabilize the mil ammo with long heavy bullets, so accuracy is horrible. Many older guns have short throats designed for 55gr and smaller bullets. While I would not be as inclined to worry about the pressures using mil ammo in a well made bolt action, still there have been enough problems reported that it is wise to pay attention to the warnings. (Many shooters will tell you that use it with no problems at all, but it only takes one time....) Personally, I don't think it is worth risking a good rifle or my ugly mug to save a few cents. The once-fired brass, however, is fine for reloading for any kind of .223. If you get a good deal on mil ammo, treat a friend with an AR to a range session and keep the brass afterward.