Isengard's Wars of Conquest

Now, what happens in the War of the Ring when it officially breaks out? Like in the main story, I think Isengard would be under severe pressure to escape the bind Saruman's betrayals created. So he would be the first to act.
Rohan is a justified first target to eliminate as it is the immediate political and military threat. It would also grant the greatest defensive depth primarily via Helm's Deep acting as a second base. Then Saruman would probably turn to the ents and Lorien to get resources and further protect his frontiers. Then, possibly the conquest of Moria with its mineral reserves would be a valuable prize. But, it seems likely that dealing with such a fortified area controlled by a population who has mastered defensive warfare and has many allies in the area would be difficult for him to pull off.
It would make more sense from Saruman to turn his attention to the Shire. The hobbits would thus be outright conquered as would the land between the Shire and Misty Mountains. This area has food resources, few enemies and possibly unexploited mineral reserves. Not to mention, this plan would grant Saruman strategic depth, meaning the loss of Isengard would not be near as damaging to him as it was in the books/movies. So it is likely that he would be taking the opening steps to take over the Shire at the same time he would be subverting Rohan. Then, only then, would he think of turning on Mordor.

The Conflict

Prelude

Now, those impacts of the subversion might be less than they were in the books and movies, but they would be present none the less. Even with the stronger impacts in the official media, an official military campaign was needed. This would likely be similar to the books. Isengard would use the Wild Men to make life hell for Rohan - seeking to distract and disperse its military forces until Rohan is easy to defeat. Meanwhile, Isengard would continue to build its army. But the discovery of Worm-tongue would likely force his hand. The White Council would be interested in countering what is clearly a hostile power on their doorstep.

Deployment

There would likely be a more straight forward approach than in the books. Due to the need to deliver a knock out blow as soon as possible, all forces would be thrown into a direct assault on the capital. The result would be a plan starting with the Battles of the Fords of Isen. Then Edoras would be assaulted. Knowing this would be coming would provoke the wise move from the books and movies of retreating to Helm's Deep. Thus, we have the set up for the climax of the Two Towers.

Battlefield

Now, this means that the main operations of Isengard were incredibly sound. The strike against Edoras and Helm’s Deep would break political control. Rohan’s capital, main fortification, elite formations and royal family would have been eliminated. Just as important is Isengard getting the Glittering Caves. Based on Gimli’s actions after the war of the Ring, we know that it has a lot of mithril and iron, critical for Isengard’s war effort. This would be critical for Saruman’s survival. Not just this, but it would provide him some strategic depth. As demonstrated when Isengard fell, Saruman is wholly dependent on it. Getting Helm’s Deep provides not just the resources but also a second base of operations.

Outcome

A great operational success would have been gained and Rohan would struggle to cope with the loss of central authorities. Yet Isengard would be confronted with attempting to occupy a vast territory. Its army would be designed for quick and strategic level victory in single battles. This same army would now be having to conduct a war of attrition against an enemy now with no single target to strike at. Meanwhile, the enemy forces would be perfectly trained, equipped and led to conduct a guerrilla war against Isengard. Isengard would be forced to disperse its forces to hunt down these insurgent bands. Agricultural lands would be in short supply in a region populated mainly by nomads. Thus, burning crops to starve them out would be much harder. Concentration of forces is the only way Isengard would be able to defeat the enemy, but such a large force would be easy to avoid for the Rohirrim. Thus, Isengard would have to disperse its forces to find the remaining soldiers. The problem is the more forces are dispersed for this task, the more vulnerable they would be to the sort of hit and run tactics Rohan is best at.

Aftermath

Now, what comes next? Saruman’s position requires that he always be on the offensive. I doubt that in the context of the world war that was the War of the Ring and his act of betrayal, the Free Peoples of Middle Earth would let him relax and consolidate his gains. As a result, he would have to move on to his next target quickly. But the downside of this is that Isengard would not have time to consolidate his gains in any sort of peaceful manner. Pacification and extraction of resources would have to proceed quickly.
The result is an occupation based on severe repression of the people and rapid exploitation of the natural resources. The local people would probably all be massacred as the armies occupy the territory or enslaved. We can expect that the need to get the minerals out of the ground fast and diversions of all resources to the army would result in horrific levels of death due to overworking and malnutrition. Beatings and executions would be common in all areas. Any resistance would be met with mass executions. This is not unheard of historically – prisoners of war were used in mines and for state projects by the Roman Empire. Looting was common in European armies until the modern day. It continues to a lesser degree even after formal, regular pay was introduced in most armies and it was criminalized. Slavery and mass executions in war time also have an unfortunately long history as well.

Included Conflicts

• Battle of Helm's Deep

Conflict Type

Military Campaign

Battlefield Type

Land