Why did Saruman attack Rohan?

[Gordis]

It is a bit puzzling why Saruman attacked Rohan when he did.

He has prepared a strong army, both on Sauron's orders and to reach his own goals. He tried to capture the Ring, fully expecting the two hobbits captured at Path Galen to have it. But he was cheated - his orcs were destroyed and the hobbits escaped. He knew he would never get the ring now.

Then suddenly Saruman attacks Rohan. When Gandalf, Aragorn, Legolas and Gimli ride to Edoras, they see smoke rising at the fords of Isen - the war has begun.

But why did Saruman attack so early? Was it on Sauron's orders? But wouldn't it have been better to attack Rohan and Minas Tirith simultaneously?

Even reckoned as a lord and captain Saruman has grown very strong. He threatens the Men of Rohan and draws off their help from Minas Tirith, even as the main blow is approaching from the East.

Moreover, Sauron was not yet ready to start the war - he only took this decision when he saw Aragorn via the Palantir.

And if Saruman attacked on his own, why did he do it? Theoden was still under his sway, held in check by Grima. Saruman faced the possibility of defeat - imagine what would have been if Rohan asked for Gondor's help and got it? The alliance worked both ways. And he also risked Sauron's wrath if he attacked too eartly.
So- why?

[rohirrim TR]

I always thought it was Sauron's orders, strategically he would of loved it if Gondor had sent aid to Rohan. Then he coud of pounced and trounced all over Minas Tirith. As for what Saruman was thinking who knows? something between overconfidence and shortsightedness I would guess.

edit: then again, he had spent a lot of time trying break down Rohan from within, the ents were a wildcard a fluke really; and if it wasn't for Eomer's loyalty he probably would have taken Rohan.

[The Telcontarion]

Well I simply thing that Saruman was not counting on the ents involvement and had them totally out of his reckoning (Gandalf said as much). If it were not for them Saruman, if he did not take Rohan and sack Edoras, would at the very least severly cripple the Eorlingas; no aid would be forthcoming to Minas Tirith from the sons of Eorl.

Doubtless this is what Saruman and his Dark lord had intended.

[The Gaffer]

I agree with El Tel: without the Ents' intervention, it would have been a very different story. It might have seemed like a tactical masterstroke to draw off support from Minas Tirith.

He had to attack well before the Minas Tirith attack or else the Rohirrim would have been long gone to the Pelennor.

[Earniel]

There is possibility, that if Saruman's army was ready he went along with his own plans of expansion, not minding whether Sauron was ready or not. Although Saruman had fallen under the sway of Sauron, he was still convinced of his own power and cunning.

He could also have hoped, by starting the war, that he could still ensnare the hobbits and the ring they carried. He knew his troops were destroyed at the edge of Fangorn. If he could conquer Rohan, he had a wider search area.

And if he had found out it was the Rohirrim who had destroyed his orcs, then maybe they had the ring, and then it was for his own sake to act as soon as possible.

[The Gaffer]

Good point. My memory is a bit hazy. At the stage of The Voice of Saruman, did Saruman still assume that Merry and Pippin had the Ring? I think that was the case.

He would fear that Theoden might take possession of it.