What if Isengard joined forces with Gondor?

I thought Saruman built his Uruk-Hai army with the intention of aiding Mordor initially, but the army itself was built by Saruman, not Sauron. So in theory Saruman could have done as he wished with his army.

[Gil-Galad]

In the book, Saruman and Sauron were not really great allies, more acknowledging that the enemy of their enemy is there friend... for now

if saruman or sauron succeeded in destroying Gondor and Rohan, they would surely be at war with each other for posession of the ring.

[Eomer of the Rohirrim]

In 'The Hunt for the Ring' in Unfinished Tales it mentions Saruman's 'treachery' to Sauron. I guess Saruman did promise service to Sauron but did he ever really mean it?

[Raynor]

I guess Saruman did promise service to Sauron but did he ever really mean it?

Indeed. In The Palantir chapter, Gandalf talks about Saruman's likely fall:

Easy it is now to guess how quickly the roving eye of Saruman was trapped and held; and how ever since he has been persuaded from afar, and daunted when persuasion would not serve. The biter bit, the hawk under the eagle's foot, the spider in a steel web! How long, I wonder, has he been constrained to come often to his glass for inspection and instruction, and the Orthanc-stone so bent towards Barad-dûr that, if any save a will of adamant now looks into it, it will bear his mind and sight swiftly thither?

and earlier, at the council of Elrond, he talks about Saruman's plans:

This then is one choice before you. Before us. We may join with that Power. It would be wise, Gandalf. There is hope that way. Its victory is at hand; and there will be rich reward for those that aided it. As the Power grows, its proved friends will also grow; and the Wise, such as you and I, may with patience come at last to direct its courses, to control it.

[Eomer of the Rohirrim]

But he always wanted power for himself and played the dangerous game of taking Sauron for a ride, no matter how scared he was of him. Wasn't Saruman suggesting to Gandalf that they should appear as aids to Sauron to give them time to find the Ring, and not seriously joining forces with the Dark Lord? That would mean that Saruman's army was never intended to aid Mordor.

[Boromir88]

I believe so Eomer...Saruman was playing a dangerous game. Saruman wanted the Ring, power, and control for himself. And I think the easiest way for him (in his mind) to get that would be to join with Sauron. It seems like a rather smart thing to do, I mean militarily the people opposing Sauron had no shot at all. So the easiest way to get what Saruman (from Saruman's POV) would be to actually join with Sauron and then betray him.

Of course these were Saruman's 'secret plans' and as he tells Gandalf; 'We can bide our time, we can keep our thoughts in our hearts...' (The Council of Elrond). Despite this I believe Sauron figured out what Saruman was up to, and Saruman really had not kept his thoughts hidden:

'I Grishnakh say this: Saruman is a fool, and a dirty treacherous fool. But the Great Eye is on him.'~The Uruk-hai

'But they shall help to rebuild Isengard which they have wantonly destroyed, and that shall be Sauron's, and there his lieutenant shall dwell: not Saruman, but one more worthy of trust.'~The Black Gate Opens

Just as some interesting side notes. When Denethor's father (Ecthelion) was the Steward; Denethor didn't like Gandalf and kept telling his dad he should trust Saruman...and in an early draft (Home VII The Treason of Isengard) Tolkien played with the idea that Boromir would side with Saruman. So, there are some loose connections with a Gondor-Isengard relationship; however once Saruman turned to evil, I doubt Gondor would have any interest in siding with him.

Originally Posted by Boromir88

Just as some interesting side notes. When Denethor's father (Ecthelion) was the Steward; Denethor didn't like Gandalf and kept telling his dad he should trust Saruman...and in an early draft (Home VII The Treason of Isengard) Tolkien played with the idea that Boromir would side with Saruman. So, there are some loose connections with a Gondor-Isengard relationship; however once Saruman turned to evil, I doubt Gondor would have any interest in siding with him.

The obvious similarity is that Denethor, Boromir & Saurman had a desire only to command; they would not serve others or let others govern their affairs. Saruman must have gained this understanding from Ecthelion that in their hearts lay an opportunity to remove Sauron through strength of arms using the Ring or other such weapon. That is what made them all fall under the lure of Sauron, & ultimately the Ring.

But then why didn't Sauron choose to target Denethor as someone to join forces with him, if he was capable of poisoning the minds of all who challenged him with the Seeing Stones?