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human
[Doctor Who] What kind of threat would it take for the Doctor and the Daleks to work together?

Is there any possible common threat powerful enough that they'd ever make a truce to defeat it?
gpt
They already have in the Asylum of the Daleks. Namely the Dalek Asylum (planet full of the Daleks too insane and dangerous for the Dalek race to put up with) was at risk of opening up and allowing the "patients" to escape, so the Parliament of the Daleks summoned The Doctor and his companions Amy and Rory to infiltrate the planet's forcefield, disable it and allow for the Dalek fleet to orbital bombard the planet to hell before the Insane Daleks could escape confinement and wreak havoc on everyone *including the rest of the Daleks*. 

Besides millions of insane Daleks, the Daleks would also agree to a truce against The Master if he was in his end-game portion of one of his *master* plans (although this would depend on the Dalek in charge, as some would stubbornly opt for oblivion rather than ally with The Doctor. 

Daleks that are unstable also would ally with The Doctor. The one in Dalek was infused with Rose's DNA and began to experience compassion, and likely would have agreed to aid The Doctor had it not committed suicide. The damaged Dalek in Into the Dalek also showed signs of compassion and recognition of beauty, and assisted the human resistance against the Daleks. But on the whole? Most Daleks would sooner kill The Doctor even if working with him is better long term. 

Pretty much anything on par with the Time War Era Timelords, The Master, a gargantuan Cyber fleet (perhaps 100,000x the size of the Dalek fleet) or anything that the Daleks cannot hope to defeat would prompt some Daleks to consider allying with The Doctor to defeat it. It's just down to which Dalek (or possibly Davros) is in control when the threat arises. Of course either way they'll try to kill The Doctor once he's hindered or defeated their mutual enemy, so it won't last long.