In Avengers: Endgame, why didn’t the Ancient One join the fight against Thanos?
The Ancient One could remove Thanos’ soul from his body to reason with him, leaving him powerless.
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The 2018 or 2023 Ancient One couldn't join, because she was dead at that time - see the events of Doctor Strange.
I'm guessing you are more interested in why they didn't bring the Ancient One from the past to the current timeline. There's a couple of things to consider:
But once again, mostly that wasn't the plan. If the plan had been to gather more firepower, they might have well fetched fifteen Hulks and unleashed them on Thanos' face.
Because Ancient One was dead by the time Thanos attacked for the first time and they had restricted supplies for time travel and also Ancient One was too skeptical about this plan too:
The Ancient One: Yes, but you’re leaving out the most important part. In order to return the stones, you have to survive.
Bruce Banner: We will. I will. I promise.
The Ancient One: I can’t risk this reality on a promise. It’s the duty of the Sorcerer Supreme to protect the Time Stone.
Bruce Banner: Then why the hell did Strange give it away?
The Ancient One: What did you say?
Bruce Banner: Strange. He gave it away. He gave it to Thanos.
The Ancient One: Willingly?
Bruce Banner: Yes.
The Ancient One: Why?
Bruce Banner: I have no idea. Maybe he made a mistake.
The Ancient One: Or I did.
And she doesn't know the future after her death so she can't leave her time unprotected as she mentions in DR. Strange :
The Ancient One: I've spent so many years peering through time, looking at this exact moment. But I can't see past it. I've prevented countless terrible futures and after each one there's always another, and they all lead here but never further.
She needs to train Dr. Strange in the future, how she can risk it? And we don't know if time stone can do time travel or not. As Ant-man time travel serum was limited.