Fighting dogs

My dog Iris is a bit "stove up" today.  That's an expression I remember my daddy using when he had done some different or unaccustomed sort of work that left him with sore muscles.  
Iris has never met a person she didn't love; she's always glad to greet a visitor and eager to be petted... she's docile and friendly.
  
With other dogs?  Not so much.  When my daughter's dog, Hawkeye, spent four days here, there was an initial tussle, but then he and Iris got along.  They weren't best buddies, but they tolerated one another, and there were no more skirmishes.
  
Cliff's sister next door has a tiny miniature dachshund, Angel.  I've kept my eye on Iris when Angel is around because I've noticed certain actions that suggested she might be aggressive toward her.
  
Yesterday Angel was in our house, playing with Iris' toys; Iris was obviously uncomfortable with that:  she gathered up all her toys except the one Angel had and put them in a heap beside her, eyeing her closely.  Still, there was no real aggressive action. 

Later on, since my daughter, her husband, and her youngest daughter were here, we went outside for an impromptu wienie roast.  Iris, Hawkeye, and Angel were outside with us.  Something set Iris off and she attacked Hawkeye; we figured the two of them could work things out.  Hawkeye is the bigger of the two, and I've never known him to shrink from a confrontation, although he will not initiate a fight.  He's a lover, not a fighter.
  
They were going at it pretty good, with Iris refusing to back down, when Angel decided to join the fray.
  
Not good.  Iris turned her attention to Angel and viciously grabbed her by the neck.  I quickly picked Iris up off the ground, and she STILL did not let go of Angel immediately.
    
There are various ways I can deal with this problem, but no matter what I do, I believe Iris will always bear watching when she has access to Angel.  Time will tell.