Aggression flare up

Question
I have an 8yr old, spayed, female chihuahua named China and a 1yr old intact, female chihuahua named Charlotte. China I adopted about five months ago and prior to my owning her she lived with an older lady, her daughter and grandson. Charlotte was adopted about two months ago from a breeder (she was kept inside for the most part and treated very well like a regular pet). Charlotte is a sweetheart, loves people, other dogs and even my cats. China is a little shy around strangers and tends to prefer being left alone when it comes to the others. When I first introduced the two they got along very well and knew when to leave each other be.

In the last week, China has become unusally aggressive toward Charlotte. A couple of nights ago, she laid down to sleep with Charlotte and the moment she woke up she snapped and bit at her. I thought perhaps Charlotte had accidently bumped her or laid on her leg so I separated the two and calmed them down. All seemed to be going well until last night. Both of the girls had fallen asleep at my feet on the couch, with about a foot between them. China woke up looked at Charlotte and started to walk toward me then suddenly turned and attacked Charlotte who was still sleeping. Thankfully, I was able to get them away from each other before there was a serious problem, but China tried to bite me when I picked her up, which made Charlotte upset (she spent a good minute barking and growling at her). Charlotte isn't aggressive toward her or anything but she both avoids China and tries to keep China away from me.  What could be the cause for this sudden change and what can I do to correct it?

Answer
Hi Helen, Actually since you only adopted China five months ago, you cannot be sure that this aggression is sudden, or if she has simply not shown it to you before this time. It really does not matter if she had the problem prior to the adoption as you can still change the behavior. It sound like the dogs are trying to work out dominance issues. China is being overly assertive and aggressive while Charlotte is being protective. It is up to you to show them both that you are the pack leader and they are equal members of your pack. This can be quickly and effectively done if you set your mind to it. Begin by taking them both for daily (minimum) walks together on the leash at the heel. Don't let them wander, make them heel and pay complete attention on you the entire time. This way you will not only exercise the body, but you will exercise the mind and convince them that they must depend on you alone for leadership and protection.
When you return from your walks, train them to work together, play together, eat together and accomplish tasks while they have already released some energy. Be sure to correct China for any dominant posturing, snarls, or stares prior to her even thinking of biting. A simple sharp noise should be enough to snap her out of it. Also pet her while she is asleep to get her used to being touched while sleeping and waking. Don't take it personal if you get bit, Helen. If you let your feelings get hurt, they can sense it and will disrespect you further. Put your mind in pack leader or strict "mommy dog" mode and do not give them an inch.  Soon they will be snuggled up under the same blanket with no worries. Leave me some feedback and follow-up if you have any other questions or to let me know how thing are going. Thank you for writing. Regards, Susan