Sudden aggression

Question
Hi there,

I have a 4/5 year old female boxer who has always had the most gentle and loving temperament. She is quite docile for a boxer too. We were given her by her last owners who couldn't keep her anymore. That was about two  years ago. Last September we moved house and got a new dog to keep her company. He is a rescue dog with a good temper too.. The new place where we live since September  is quite remote and there aren't as many dogs being walked in the area. The last few times we have walked the dogs though and met other dogs, Levi has chased and attacked them - snapping growling and biting. These are dogs she has met before and was ok with previously. The dogs are usually smaller than her..
Before we only had the problem of her running up happily and scaring other dogs. The other rescue dog we have  is a boxer staffie cross and he scares other dogs by being very happy and boisterous too. Sometimes Levi is jealous of him so we give her lots of attention. I hope I have given enough information, we would love to know how to stop this behaviour so we can let her off leash when were walking her. She is spayed since April 2010..There haven't been any other changes that I know of that would affect her...
Thanks for your help
Aoife
The boxer- Levi is spayed for the last year.

Answer
First, no off-leash! They have not been properly trained, and they are not stable or predictable, therefore, may harm someone, run off, get injured, etc. It would be irresponsible of any owner to allow either off-leash-- ever.

The female Boxer is displaying signs of being territorial and protective; the male is even more unpredictable being a rescue; which you do not know his previous history.

Boxers can be extremely jealous, especially ones that have been insecure being in many homes; she will protect what she has including love and attention as well as her territory (home and areas), food, toys, and the like.

Having been in a different home, and him being rescued, means they come with baggage and insecurities.

Owners willing to rescue and take on any dog(s) with prior owners and histories (even worse-- the unknown history), must accept the quirks that they come with and be patient and willing to modify their behaviors.

You need to establish your position in the pack (them and they family is a pack) as the alpha-- not her trying to be the alpha (female Boxers are more aggressive than the males)...

Do this trick for a month to prove you are the alpha of the pack:

(with both dogs)

Place their food bowls on the table or counter with food in them.

Place cookies or crackers on the table or counter BEHIND their bowls to appear as though they are being retrieved from the bowls...

Stand and eat the cookies or crackers while NOT looking at them. Take your time......

When you are finished, place their bowls down on the floor and tell them they can eat.

The alpha always eats first and all he wants while the pack watches and waits, then the alpha gives the pack permission to eat when he if through.....

Do this faithfully for a month.

Also, when she (and/or he too) act and show aggression, immediately put her or him to the ground, down into a submissive position until they submit to you. Hold her or him down for a minute or so.

Be consistent with this each time unacceptable aggression flares....

In the meantime, while walking them on their leashes outside, when one becomes aggressive or distracted by another dog, stop immediately and turn the dog facing you and give a treat to him or her........ this distracts them and they will soon associate another dog passing by as a time for a treat and attention instead of being focused on their insecurity aggressions.