Yellow Labs aggression

Question
I have a 4yo Yellow Lab. We got him as a puppy, had him neutered, took him to puppy obedience class, etc. When he was 2 we rescued a like aged Beagle, they have been together for 2+ years now and just recently, within the last month or so, the Lab has started to attack the Beagle. It is not over food (they eat together with no problem) it just seems to happen for no apparent reason. We had to take the Beagle to the vet because the fight caused a lot of injury him. This morning we were together, myself and both dogs, for about 30 minutes when my Lab went after my Beagle again. I pulled the Lab off, separated the two and now I need to do something. Any suggestions would be deeply appreciated. Why would my Lab suddenly turn on my other dog?

Answer
This is a difficult and complex problem.  Usually if there is going to be be trouble, it will start before 4 years old.  Many people are denying the concept of dominance, but they don't seem to have a good explanation for such behavior or a solution.  As the dogs have matured, one of them is insisting on being dominant, and the other is resisting.  Some of the the signs are subtle, body position, butt sniffing, eye contact, etc., and hard to recognize.  The Beagle could be bringing it on himself.  He is neutered too?  

What you need to do is take over as top dog.  Top dogs aren't drill sergeants, but more like old fashioned, strict, but loving parents.  The puppy class should have taught you that.  Unfortunately, too many classes are about the puppy getting a treat if it sits.  Try a book, How to be Your Dog's Best Friend, by the Monks of New Skete, Don't Shoot the Dog, by Karen Pryor, Dog Problems, by Carol Lea Benjamin, HELP, Mt dog Has an Attitude, by Gwen Bohnenkamp, Owners' Guide Better Behavior in Dogs and Cats, by William Campbell, What All Good dogs Should Know, By Wendy Vollmer, Puppy Primer, by Brenda K Skidmore and Patricia B. McConnell, Ph.D., Beginning Family Dog Training, by Patricia B. McConnell, Ph.D., 1996

Even then you may not be able to solve the problem.  You could look for a private trainer, one APDT certified, but not too much into psycho babble. Unfortunately, in a few cases the only solution is to rehome the one dog.