Our 12 year old dog is being attacked by a new 12 month old Great Dane. What can we do?

I have a 12 year old Kuvasz, who has governed our pet family for the same amount of years. We recently introduced a Great Dane into the picture. The GD is slightly over a year now and has showed dominance over the Kuvasz by actual fighting. The Kuvasz has ruled by intimidation. The Great Dane didn't buy it and challenged her. The result was bloodshed in the form of puncture wounds. I tried to establish that the Kuvasz is dominant over the Great Dane, by feeding, greeting, and preferential treatment. It worked for 3 months. Today all dogs were outside and when I came home my husband informed me that the Great Dane was on top of the Kuvasz. The Kuvasz was trying to get away. She has serious bites over and under her eyes. I don't think the Kuvasz is going to live much longer she has slowed down and is starting to smell different. What is going on. I know that from now on I will have to separate them. The Dane doesn't do this to our other dog who is a neutered male. Besides, being my fault is there anything else I could do to quit my Dane from this behavior in the future. I do take her to obedience classes as well as handling classes. I am interested in any help you could offer.