For a Border Terrier the age of around 3 seems to be the age they really do become Border Terrorists. (This is our opinion not a breed confirmed trait).
By this we mean your Border Terrier may suddenly decide it doesn't like other dogs, for no apparent reason, while out on walks for example.
This is due to the grumpier side of the Border and most Border owners will experience something like this at some point.
You need to be looking ahead and behind you on your walk constantly to make sure you see the dog which may set your dog off barking and pulling etc before they do. Once you have spotted the dog you need to gain your dogs attention continually until the dog has passed you. You may use the 'heel' command whilst also ensuring your dog is watching you and not their surroundings. If you need to have a small treat or a favourite ball in your hand to keep your dogs attention, then do so.
Reasons for this aggression towards other dogs could be:
Do not dispare, with lots of hard work and patience on your behalf you can correct this behaviour.
As soon as the other dog has passed you by, you have kept your dogs attention and your dog has behaved as you wished with no barking or pulling on the lead give your dog a reward and praise them.
This sounds simple, we know, but in reality it is
not that simple. Your dog will see the dog you are trying to distract them from and they will ignore you and they will bark but with hard work they will soon realise that barking and pulling gets them nothing but ignoring the other dog will result in a treat or their favourite toy.
You need to ensure all the family members who walk the dog are following the same procedure and using the same words and techniques to reinforce the behaviour you require.
Border Terriers are, of course, very clever, you may find once your dog has realised ignoring the other dog results in a treat that they will take the behaviour too far the other way, and expect a treat after every dog that passes them, even the ones who they are friends with and would never bark at, in this instant praise them for being good but if you can get away without a treat do so, otherwise your dog may have had the equivalent of a meal by the time your walk is complete if it is a busy dog walk.