What if Isengard joined forces with Gondor?

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?