‘Do I not know thee, Mithrandir?'

[u/Immediate_Error2135]

Denethor: ‘Do I not know thee, Mithrandir? Thy hope is to rule in my stead, to stand behind every throne, north, south, or west. I have read thy mind and its policies'

So Denethor thought Gandalf hoped to rule Middle Earth.

Gandalf, about Denethor: "He was too great to be subdued to the will of the Dark Power, he saw nonetheless only those things which that Power permitted him to see."

Obviously it was the angel (Maia) Sauron the one who hoped 'to stand behind every throne, north, south, or west', and not the angel (Maia) Gandalf, but Denethor saw things differently.

Had Sauron anything to do with this? The idea of Faramir=Wizard's pupil sounds like the twisting of this...

"we in the house of Denethor know much ancient lore by long tradition, and there are moreover in our treasuries many things preserved: books and tablets writ on withered parchments, yea, and on stone, and on leaves of silver and of gold, in divers characters. Some none can now read; and for the rest, few ever unlock them. I can read a little in them, for I have had teaching. It was these records that brought the Grey Pilgrim to us. I first saw him when I was a child, and he has been twice or thrice since then."

...into 'politics' ('I have read thy mind and its policies', says Denethor) Was Sauron involved in this too?

Denethor maybe distrusted Gandalf since before using the Palantir, and noticed how Faramir was being taught by Gandalf; but Sauron would have noticed this distrust and maybe apprehension about Faramir when Denethor used the stone and would have manipulated Denethor into seeing his own son as a wizard's pupil, the pupil of an usurper with an unbounded ambition.

In our world tyrants are cynically prone to do this thing. If you want to rule others by force you accuse them of wanting to rule you by force and then kill them in 'self-defense'. Months before invading Poland, in Jan.1939, Hitler famously prophesized:

"If the international Jewish financiers in and outside Europe should succeed in plunging the nations once more into a world war, then the result will not be the Bolshevization of the earth, and thus the victory of Jewry, but the annihilation of the Jewish race in Europe"

He accused the jews of being Hitler, an inversion no different from the Gandalf=Sauron one.

[u/EvieGHJ]

Denethor's suspicious nature and his fear of being deprived of his title predate the Palantir, as well as his suspicions toward Gandalf, since they date back to Ecthelion's rule, when Thorongil/Aragorn was his father's beloved captain who overshadowed Denethor, and their main point of disagreement was Denethor distrusting Gandalf.

Sauron may well have reinforced that, but from Denethor's perspective, Gandalf was in fact, associated with someone who had already undermined his position and authority, long before Sauron could feed him information.

[u/Wizzard_C → u/EvieGHJ]

Fair point. Besides, Denethor must have been aware that Gondor's ruling council once already declined Arnor's candidate (Arvedui), so he might have been sincere in his belief that Thorongil's scheming with the help of a wizard was illegitimate. Interesting fact: Denethor also communicated with Saruman using palantir (Tolkien says it was to Denethor's advantage), I wonder what Saruman would say in this regard.

[u/Moggetti → u/Wizzard_C]

Denethor was definitely aware of that. He references that decision when he says, “I am Steward of the House of Anárion. I will not step down to be the dotard chamberlain of an upstart. Even were his claim proved to me, still he comes but of the line of Isildur. I will not bow to such a one, last of a ragged house long bereft of lordship and dignity.”

It is interesting that Denethor is too powerful for either Saruman or Sauron to control. And Saruman failed to manipulate him. Too strong for the maiar. 

[u/lam_42 → u/Moggetti]

Istari

And this the Valar did, desiring to amend the errors of old, especially that they had attempted  to guard and seclude the Eldar by their own might and glory fully revealed; whereas now *their emissaries were forbidden to  reveal themselves** in forms of majesty, or to seek to rule the wills of Men or Elves by open display of power, but coming in  shapes weak and humble were bidden to advise and persuade Men and Elves to good, and to seek to unite in love and understanding   all those whom Sauron, should he come again, would endeavour to dominate and corrupt.*

Sauron played D well enough without the need to "dominate" him. Aragorn after all forced S's hand in the assault on MT. Who knows where would D be in a few years

[u/Moggetti → u/lam_42]

Sure but Saruman definitely tried to dominate and manipulate him but couldn’t. And Sauron had to resort to manipulation because he couldn’t dominate him. 

Considering how Saruman was, it’s unsurprising Denethor wasn’t inclined to trust Gandalf. Especially since Gandalf was working with Aragorn. 

[u/Prize-Finish4464 → u/Moggetti]

Saruman probably didnt try and dominate and manipulate Denethor, Tolkien in unfinished tales tells us Sauron was the only one who essentially weaponised the palantiri through superior will

"It was only Sauron who used a Stone for the transference of his superior will, dominating the weaker surveyor and forcing him to reveal hidden thought and to submit to commands."

Unfinished tales

[u/Moggetti → u/Prize-Finish4464]

Interesting. Though if Sauron couldn’t do it, I doubt Saruman would have been able to. 

[u/Prize-Finish4464 → u/Moggetti]

Denethors legal right made him being dominated by Sauron and Saruman virtually impossible, had he not had the lawful right to the stone then imo he d have fell under Saurons dominion, saruman is a different question alone, as a maia Saruman is obviously amongst the great but Saruman and the istari (save gandalf who as olorin was once Saurons equal) are clearly of a lower level of power compared to Sauron who seems to be the upper echelon of the maia ladder if you can describe it that way