one of my rescue lhasa apsos shows aggression to other dogs i want to let him off his lead in the park or woods but if he sees another dog in the distance he tears off after it going deaf to my calls runs up and around it snapping at his face which of course turns the other dog to bark and snap at him ,he is very small and any big dog could kill him in one sweep ,walking in the park or street when he sees another dog he twists and pulls on his lead growling and snarling to get to it ,he met friends of mines dogs a couple of weeks ago they are the same breed at my house after keeping him on a short lead in the garden for an hour i was eventually able to let him off and they became best friends playing for hours, last week he met my brothers dogs at his house and at first he was going crazy but again after an hour all was calm .
yesterday in the park all was fine he was off the lead and socializing with other dogs then walking round the corner a staffie was sitting by its owner minding its own business when my dog flew at it snapping at its face !
The problem you have here is that the dog needs to be seen to be assessed. I could offer advice but am not going to as aggression is a tough one to deal with and I feel your best route would be to have a few sessions with a behaviourist. Ask your vet to recommend one in your area.
Do not let your dog off the lead wherever there may be other dogs until you have sorted this. Its not fair to other dogs or to him. Enrol for training classes to get him socialised and teach him to come when called. This is an accident (no, that should be incident) waiting to happen and if he ran up to one of mine and did it I would not be very pleased to say the least. As Lisa says, perhaps a behaviourist would be able to advise but you need to do some work yourself with him.
Would it be worth going back to the rescue organisation to ask a bit more about his background? There is almost certainly some past experience at the root of this. Why did he end up as a rescue dog? If there is a root cause, at least you'll have something to work on.
First point of call should be the rescue who should offer backup for their rehomes. After that if no joy get the dog assessed by a behaviourist, a member of apdt
http://www.apdt.co.uk/
if anyone starts spouting dominance theory leave imediately