Yep, it's me again, and I need to just confirm exactly what is "normal" in the way of aggression on the part of my 20 month old GSD male, Luke.
A while back I had a real scare when he damaged a male pit bull at the lake. This was NOT typical for him, he is very laid back, but not in any way timid, he is not sensitive to much at all, just stays calm.
I realize that as he matures his temperment is going to stiffen up a bit and we have made sure to correct things before they become problems.
I recently found his breeder and they thought it would be ok to bring their male over, he is a gorgeous 8 year old male, who is pretty laid back like his progeny Luke. Anyway, something clicked after our discussion so when they arrived I said lets introduce them at the fence. They were fine for about 45 seconds before one growled at the other and from then on it was nuts. I had to physically catch Luke in mid-air to keep him from going over the fence, then he tried to go under the fence. He did down finally, but it was no cake walk getting him to stay there. I put him in the house pretty quickly.
I know that it is "his" yard, and that makes a big difference, but I am not completely sure exactly how he would have reacted in a different setting, given his burst of dominance at the lake a few weeks ago.
Call me an idiot, but I need to get this under better control. HELP?

Just a status; Took Luke to play with one of the K9s here on base that he had played with before, went fine. Then took him to play w/ a Doberman male that a friend of mine has that he hasn't seen in awhile, they were fine. I have been doing some extra work on making sure that his down/stay is a little more solid than it has shown itself to be in the recent past too.
I mainly just had a bit of consternation as to the "exact" causes of these bursts of dominance. Was there something that I was not seeing correctly or what? Appears to be normal. The handler onbase confirmed a few things for me too about what his adult male will tolerate, and what it will simply "not" tolerate from another male. Luke's behavior was outstanding in both situations in which there were no territorial issues involved. As for the lake incident I just have to chalk that one up to the other dog growling and Luke taking matters into his own teeth. It showed me directly that he was not as under control as I thought that he was. As for his "sire" incident, that was simply not a good idea from the outset. However laid back they both are, they are still large, dominant, intact males.

There are so many factors to consider that it boggles the mind. One that does stand out to me is that your dog is 20 months old. I've found that if aggression is going to be a problem in a normally non-aggressive dog it will happen when he's going through his puberty phase. In my dogs, when I notice they are starting to sort of feel their masculinity come on I try not to socialize them with other males until they are done maturing. In my current dog, this started at about 11 months. I don't envision this ending until he's close to 20 or 24 months old. I still socialize him with other dogs, but use females.

Another thing to consider is the other dog. If the dog you are socializing with is a confident alpha male then he won't be very threatened by another dog. Also, if your dog is comfortable being a subordinate then they should get along. But, if your dog decides to try to move up the ladder or the other dog is also a subordinate or not a stable alpha then their will be some rank determination going on.

But, like I said, there are so many things to consider. In my opinion, you can either avoid situations that could result in aggression or confront them but get the help of an expert to watch and figure out what's really going on.

As a side note, I'm a Military Working Dog handler. You mentioned your pet playing with a base K-9. If this was an MWD, it should not be allowed; especially if your dog had been aggressive in the past. If an MWD gets bit or bites it will end up quarantined for 3 days. That's a $37,500 asset (last I checked for a patrol/bomb dog) that the base won't be able to use until it's healed or off quarantine. Sorry to sound so stern but sometimes handlers don't realize how important the safety of their dog is to the protection of the installation.

He is definately a dominant type, but he is not goofey about it. He has been playing w/ that particular GSD/MWD at the base since he was about 9 months old. The MWD is going on 6 yrs old, really nice dog, but he is a real dominant beast too, I think that Luke just defers to him and they get along beautifully. That dog has never been at the house here though either, as they have played at the lake, or in a field by the runway. I will think twice about letting him play w/ him again though, just because I don't want my friend to get himself into trouble.
I have been just working Luke's obedience alot more than I had been in the month or so prior to that incident at the lake. I still run that whole thing through my head and I just don't see how it could have went any differently, even if I had put Luke into a down/stay and he did so, it would simply not have meant much the second that the pit did what it did. Luke trusts me, and I'm simply not going to let him get hurt while he is listening to me, whatever it is is just going to have to brawl the both of us.