Did Saruman make the same mistake as Sauron?

Question

By attacking Rohan so early, why Saruman attacks Rohan with his entire force at the time he does confuses me a lot. Not only has his Uruk-Hai band not brought back the halflings he desires highly but they have been completely destroyed. Now if I remember rightly, Gandalf says to Aragorn news of Ugluk's demise will not reach the ears of either Isengard or Mordor, but Saruman being the wise fellow he is, must surely be able to put 2+2 together and realise that the horse-lords have defeated his secret band of Uruk-hai that set out to Parth Galen.

So now he has no halflings, nor a ring (we know that Frodo in fact has the ring, but Saruman thinks Merry and Pippin do), and his chieftains Ugluk and Mauhur have both been defeated. Saruman at first was playing his cards perfectly, he sent a small but great enough troop of orcs to capture the hobbits, which they do, and in fact kill the Steward of Gondor's son in the process; he had strong enough forces placed at the fords of Isen and then after the failure to get the ring, Saruman being the wise guy he is, throws his entire force at a Rohan leaving none to defend Isengard!!

Is there any reason that might justify why Saruman attacked so early?

Answer 1

It's actually the opposite mistake, as Tolkien notes in "The Battles of the Fords of Isen" (published in Unfinished Tales, my emphasis):

When Théodred was at last slain Saruman's commander (no doubt under orders) seemed satisfied for the time being, and Saruman made the mistake, fatal as it proved, of not immediately throwing in more forces and proceeding at once to a massive invasion of Westfold.

Tolkien then goes on to note what would have happened if Saruman had acted earlier:

If the invasion of Westfold had begun five days earlier, there can be little doubt that the reinforcements from Edoras would never have come near Helm's Deep, but would have been surrounded and overwhelmed in the open plain; if indeed Edoras had not itself been attacked and captured before the arrival of Gandalf.

In an author's note to this essay Tolkien then goes on to discuss the Ents:

The Ents are here left out of account, as they were by all save Gandalf. But unless Gandalf could have brought about the rising of the Ents several days earlier (as from the narrative was plainly not possible), it would not have saved Rohan. The Ents might have destroyed Isengard, and even captured Saruman (if after victory he had not himself followed his army). The Ents and Huorns, with the aid of such Riders of the East-mark as had not yet been engaged, might have destroyed the forces of Saruman in Rohan, but the Mark would have been in ruins, and leaderless. Even if the Red Arrow had found any one with authority to receive it, the call from Gondor would not have been heeded – or at most a few companies of weary men would have reached Minas Tirith, too late except to perish with it.

The First Battle of the Fords of Isen was on 25th February, according to the "Tale of Years" (Lord of the Rings, Appendix B) and this was the battle in which Théodred was killed, and therefore obviously the one referred to in the first quote above.

The Second Battle of the Fords of Isen was on 2nd March ("Tale of Years"), or 7 days later (February in the Lord of the Rings calendar for that year had 30 days), in which Erkenbrand was defeated. Saruman's main invasion of Rohan must have begun on or before that day, because the "Tale of Years" also notes that the Ents reached Isengard on the night of the 2nd.

The Battle of Helm's Deep began on the 3rd of March, the same day as the Ents completed the destruction of Isengard.

Going by these dates, the "five days earlier" that Tolkien refers to above must have been the 26th or 27th of February; notable dates for comparison with other events are (all from the "Tale of Years"):

• 26th February: Breaking of the Fellowship, capture of Merry and Pippin.
• 27th February: Éomer pursues the Orcs.
• 28th February: Éomer overtakes the Orcs.
• 29th February: Merry and Pippin escape and meet Treebeard, the Orcs are destroyed.
• 30th February: Entmoot begins, Éomer meets Aragorn.
• 1st March: Entmoot, Aragorn meets Gandalf.
• 2nd March: Healing of Théoden, end of Entmoot, Ents march on Isengard.

An earlier invasion of Rohan would therefore have obviously disrupted all of these events.

Answer 2

There are few moments we have to consider:

1. Being under the Sauron's influence Saruman's main goal was to prevent Rohan from helping the soon-to-be besieged Gondor.
2. Saruman's deception with Grima failed - Theoden is no longer under his thumb, thus potentially allowing Rohan to defend itself and(or) help Gondor.
3. He has no real information about his attempt to capture the Ring. And if something had gone wrong then Ring could have easily escaped from his grasp due to the nearness of Rohan and Gondor.
4. Saruman still thinks that Rohan forces are weak due to the earlier inactions of Theoden and Saruman's forces raids, so a swift "blitzkrieg" can easily decapitate the kingdom's leadership.
5. At the same time his own troops are numerous and well prepared.
6. He knows nothing about Ents. Well, he knows, but he considers them to be either too indecisive to fight for their forests or too far removed from human affairs to care about anything.

Basically:

• He has to stop Rohan from restoring its forces (both for his own reasons and Sauron's orders)
• He is very anxious about the Ring - what if the troops that "captured it" were taken out by Rohan and the Ring is now somewhere there.
• He lost the initiative due to Gandalf's healing of Theoden, so he must retake it.
• His enemies are weak and he is strong.
• He do not see any other threat for his plans.

His only real mistake was not taking into account Ents (because even the relief force with Gandalf wasn't very strong to actually defeat Uruk-hai with Dundelings, it is the Ents who really scared them with Huorns cutting the way back, it is the Ents who captured Isengard). Unfortunately this mistake was a fatal one.

Answer 3

This is just speculation, but I wonder if Saruman knew that the orcs sent to capture the ring bearer were defeated. Perhaps he thought that after rescuing the hobbits, the Rohirrim had gotten hold of the Ring. It would then make sense to launch a full assault on Rohan. If Rohan has the Ring, then (in Saruman's mind) they can easily over power him. Whereas if he can successfully defeat Rohan and capture the Ring, then he can also defeat Sauron. Basically, he needed to attack Rohan, and the outcome would make or break him.

Answer 4

I think this was simply because he believed Sauron was his ally, whereas the truth is, that Saruman was just Sauron's tool just like Gandalf tried to warn him.