fear aggression

Question
I have a 2 year old female cocker spaniel desexed (Comet) she sometimes exhibits fear-aggression when confronted by other dogs and occasionally people if she feels that they may be threatening.  The problem is that they are not necessarily doing anything that is threatening, but if she feels they are, she will generally bark and snarl at them in an attempt to scare them away.  She has not bitten any dog or person and seems just to
want to show them that she is in control.  So long as the other dog/person does not react aggressively, she generally settles down and this is the last of it.  This problem seems to be worse and mostly happens when she is on a lead.
Comet is also on Clomicalm to help reduce her anxiety level which seems to be helping.  Other then this, she is the perfect dog.  Importantly, she has never shown agression to anyone that she knows.  So it is just the initial introductions that are difficult.  

Answer
The key to most behavior problems is approaching things using the dog's natural instincts.  Dogs see all the people and dogs in the household as a pack with each having their own rank in the pack and a top dog.  Life is much easier if the 2 legged pack members outrank the 4 legged ones.  You can learn to play the role of top dog by reading some books or going to a good obedience class. A good obedience class or book is about you being top dog, not about rewarding standard commands with a treat. Start at http://www.dogsbestfriend.com/

With you as top dog and in charge, it then becomes your responsibility to protect her from the scary strangers.  She might become more trusting of strange people if she discovered many of them are carrying treats for her.  Treats do not need to be large or anything special.  If the dog doesn't get many treats, even a piece of her regular kibble is enough.