Duke does not display aggressive behavior outside if Trouble is not with him.
The scenario went like this - took Duke and Trouble out for a potty break as always Duke on leash, Trouble off.
Everything is fine. Duke looks up and discovers on the other side of the yard, (close to Trouble) was a smallish yellow dog. Barking, tugging, let me eat it Duke comes out. Small dog leaves, Duke calms, we all go inside.
Duke begins to whine, I figure his potty break was interrupted, so I take him back out by himself. Little yellow dog returns. Duke watches it quietly, watches, watches, watches as it crosses the yard, our neighbors yard, and continues down the street behind us. All the time Duke is quietly watching, I am praising him.
Our thoughts at this point is his bad behavior outside is protecting (guarding) Trouble. We know he protects her because if we correct her, he will get between us and her.
Need trainer comments please.