St. Bernard Agression towards other dog

Question
Hi.  I have an 18 month old St. Bernard who we have had since a pup.  We also
have a 12 year old border collie and lab mix who we have had for 12 years.  
Sid, our 12 year old, is the sweetest kindest most well behaved dog you will
ever meet.  Jazzy, our Bernard, is also sweet and lovable and well behaved and
both are wonderful with our 3 children.  Sid is getting older and is definitely
slowing down.  My problem is our Bernard is becoming agressive.  Many days
nothing happens, but for instance just this morning our Bernard seemed to
almost stalk Sid and growl and then pounced on him.  Our Bernard gets
extremely agressive and had Sid by the neck growling and choking.  There
wasn't any blood and Sid does try to fight back but it just seems to be getting
worse.  For the most part when our Bernard starts to stalk Sid kindof cowers
and lays down and really doesn't reciprocate the agression-he just wants to
be left alone.  We try to break it up before it happens by distracting, but once
jazzy gets in that mode she really can't be stopped.  Then later in the day
they will be playing great.  I am just concerned about Sid-he is so old and
doesn't deserve this-it is just breaking my heart that he has to deal with it.  
We love both dogs, but are really worried that Jazzy is going to hurt, or even
kill our older dog.  Do you have any ideas or ways we can deal with this?  
Thank you for your help.

Answer
I think you have legitimate cause for concern. Animals (dogs) are opportunistic predators and your young adult female is seeing an opportunity. Not good! I had a pit bull that lived to be 17, the last two years of her life I never left her alone with the other dogs. They never did anything to her but I just felt a need to keep her safe from getting knocked down etc.

Your old guy needs a safe place too. Your young female needs to respect the boundaries that you set for her regarding him.

Do not lull yourself into the pattern of "all seemed OK for many days. It appears to be escalating.. You may want to consider having someone come out and evaluate in home. Try ccpdt.com"