Could female hormones be to blame?
I had just arrived. Without warning, Bella came through the door and attacked Poppy. Fortunately she was wearing a harness and with very fast reflexes the lady lifted her off the floor and onto the chair.

Bella
I thought I had carefully choreographed my arrival. Knowing Bella’s aggression towards older miniature Dachshund Poppy had been escalating over the past few days, I was to settle in the room with Poppy – they would have her on lead. Then, also on lead, Bella was to join us.
Unfortunately it happened too quickly and the two dogs met in the doorway and Bella exploded.
The final trigger at that moment, I believe, was that my arrival had pushed arousal levels up sufficiently to send Bella’s ‘stress bucket‘ overflowing.
Is Bella working up to her season?
The aggression from the younger Bella to Poppy may well have been hanging in the air so to speak for a while. Is she working up to a season? Female hormones may be to blame. Both dogs are entire.
Something, however, must have triggered the sudden very recent escalation. We know it’s connected to arousal, but why has Bella waited until she is three years old? They used to be together all the time and sleep in the same bed.
For some reason stress levels are through the roof just now. Over the last days both little dogs have become a lot more reactive to everything.
Being weaker and wary seems to make eleven-year-old Poppy a target. Dogs aren’t always sympathetic (I don’t actually know if they do ‘sympathy’ in the way that we do) but a yelp of fearfulness or pain can often trigger an attack from the already dominating dog.
Christmas could have played a part, with lots of people and excitement. Apart from that, the most likely culprits are either female hormones or pain – or both.
The first important thing I advise them to do is give Bella (the instigator) a thorough vet check and they can discuss spaying her. What effect could female hormones be having on her attitude towards Poppy? She does all she can to dominate her, she always covers her pee and she stands with her head over her back. She has also started to mark in the house too. She’s not a happy dog.
Preventing further rehearsal
Another vital thing just now is to prevent any future episodes though management. Whether female hormones are involved or not, with each occurrence the next one becomes more likely and more intense. Whenever anything arouses them now, Bella redirects onto Poppy.
When two females start fighting, it can be hard to come back from.
The couple will do short set-ups with both dogs on lead in the sitting room at a distance while they work on positively reinforcing each dog for looking at the other. We began this for a couple of short sessions while I was there (Clever Bella soon realised that looking at Poppy earned food, but that’s okay).
From now on, they must only be up-beat and encouraging whilst making quite sure Bella is unable to get close enough to Poppy for trouble. No warning ‘uh-uh’ or scolding.
We discussed ways of taking both dogs outside at the same time – it would need two people. They will take Poppy out first, both dogs on long loose leads and always keeping good distance. It’s always best that the dog most likely to be territorial is the last out.
Food-foraging (Sprinkles) on leads can be a companionable activity that the two dogs can safely do together when ready.
Walks can be approached in the same way – Poppy at the field joined by the other at a distance, then parallel walking, only getting as close as both dogs are comfortable and being ready to jog off in another direction with Bella if she shows any tension towards Poppy.
Priorities
Summing up their three initial priorities:

Only a couple of weeks ago
*A thorough vet check for Bella and discuss the affects of female hormones and spaying.
*Managing the situation so that no further fights or attacks are possible.
*Reducing all stress and arousal that they can. We found a number of things that they could do a little differently that, when added together, will help.
This is a great blog by Adrienne Farricelli, Why Are My Female Dogs Fighting?
To spay or not to spay? Are female hormones the culprit? That is the question. (Please don’t think neutering is a quick fix for behaviour problems. It can sometimes make some behaviours worse). Every case has to be taken individually and in context.
In Bella’s case I feel it’s well worth trying. She now marks in the house, she always has to be last to pee and she has increasingly shown dominant behaviour towards poor Poppy.
They will give it a couple of weeks doing everything else suggested, and if no improvement then spaying Bella is the remaining option.
Update: The vet has ruled out pain. Possibly a deeper check could be done later.