Raid on Dunharrow

Using the formula I used in discussing Edoras, I think Isengard would Dunharrow with 200 wild men and 200 warg riders. The main difference would be with Rohan. We know that at least one village, possibly more are in the area. Because of the role of Dunharrow in mobilizing Rohan's forces, those that live in the area would probably unofficially gather at Dunharrow prior to marching to Helm's Deep. Rough guess here is that between 3-5 would be in the area and another 1-2 on their way when Isengard's troops show up.

I suspect that Isengard's plan here would be pretty much the same as at Edoras. Charge in, surround it and burn it to the ground with rockets. Then massacre anyone who runs out of the village. But the village would be only of secondary importance. If it could be bypassed, it would. I doubt Isengard has too much of a problem with a random village as such.   But he would care about the Dunharrow amphitheater area itself. This is the main defensive area and the area of strategic importance to Rohan. So Isengard would most likely send the extra wargs straight up to the top in a blitz. Followed with the Wild Men. The "Rocket Wargs" would stay behind to handle the massacre.   The Wild Men need to be part of the attack on the amphitheater itself. Basically, it is one of their holy sites. So being a leading force in retaking it would go a long way to solidifying their loyalty to Saruman. Plus, taking here would be one of their major war aims.

The splitting of forces would prove fatal though. Remember, 360 to 600 Rohirrim would be already here. Then, there would be the the 120-240 that would be on their way. Which means that Isengard's forces would be split into groups too small to resist. Then they would be attacked from two directions with a enemy position in the middle. Said position would be either burning or hostile to them. At best, it would prevent the sort of defensive activities Isengard would need to survive. At worst, it would be actively killing Isengard's troops.

Basically, the battle would leave parts of the village damaged. The villagers would probably then be moved up to the amphitheater into a temporary camp. Because it is clear that Isengard would be attempting to take the area, one of the companies would stay behind to defend the area. The rest would march on the Helm's Deep. But the threat would primarily be over. The whole force from Isengard would be destroyed, probably to the last soldier.

In real life, perceived internal enemies allied with hostile powers have been the targets of genocide. Usually there is preexisting hatreds. Then all it takes is a few provocations the militant bigots can interpret as pretext and atrocities will occur. When this is combined with a sense of national crisis, things can jump to full scale genocide. People will see these internal "enemies" as directly attacking the nation and due to the crisis, so called half measures would not be justified. Plus, in war time you see a larger number or people trained armed and instructed to kill enemies.   The participation of the Wild Men would prove dangerous. Racial tensions between the two groups had been building for awhile before Isengard got involved. Aiding a major hostile power like Isengard would only add to the tensions. Them helping said hostile power burn the capital would also be pretty inflammatory.Then, the participation in the murder of civilians at a place central to Rohan's national myths and culture would probably be the final straw. The Wild Men would probably become the victims of a full genocide.   The Rohirrim would continue to mobilize. This would provide the mechanism for spreading the news and often many damaging embellishments in the retelling. As the troops move throughout the area, the troops would begin to seek vengeance. This would probably involve massacres of Wild Men as they are found.   After the main fighting against Isengard is over, things will begin to settle down a bit. Troops will be sent to fight Mordor. Plus the crisis directly threatening Rohan would have lifted. This would allow the survivors to more easily flee west of the Misty Mountains, out of Rohan's reach before the troops return. But major losses would still have occurred.