Interventions in Easterling Civil War

One element that led to those successful independence movements in the Russian Civil War was outside help. Germany had great interest in exploiting the collapse of its former enemy. So it used its power to essentially impose independence of these regions on Russia. France in particular was also interested in a "Cordon Sanitaire" - an alliance of the newly independent states to contain both Soviet and German power.
Renegade Freikorps didn't help matters either. These troops were technically under German command. But enforcement of German control over them seems a bit of a joke. These forces were fighting primarily in the Baltic Sates to protect the privileges of the local German populations, who formed essentially the local aristocracy. They also wanted land for themselves after the conflict.
Then there was the French and British involvement in the Russian Civil War. They both sent troops, but most of their involvement came in the form of aid to the conservative "White" factions. The Whites were quite a diverse group in real life, but there were some commonalities. First, they wanted a return to the old borders of the Empire, so the pro-independence groups for the nationalities on the fringes were hostile. They were also primarily former Tsarist officers and conservative land owners. For our purposes though, they were not seeking a return to the old regime. Instead, they wanted a return to the post February revolution regime of a liberal, moderate constitutional parliament. Basically the traditional freedoms like free speech, but not social restructuring. This is in line with what I think the elves would want to see in a new regime.

The Conflict

Prelude

We can see this play out similarly with the Easterlings. Gondor, Mirkwood and the Lonely Mountain would all want some form of buffer between themselves and the Easterling central government, whatever replaces it. I think the Lonely Mountain would fill the role of Germany. It would directly invade to take territory deemed valuable for its economic security. Then, the same raider/mercenary forces would seek to exploit the chaos for their own interests. Similar to the freikorps.
Which means the Lonely Mountain would directly intervene to set up a network of client states to serve its economic interests. Colonists from here would attempt exploit this to carve out their own areas. Gondor would support the Easterling ethnic groups on border with military hardware, some financing and limited troops. Mirkwood would step up support for the Auxilia Brigades attempting to solidify a new regime.
Gondor would seek the Cordon Sanitaire. It would be incredibly beneficial to see this power stripped of territory and a buffer area created to keep the new regime away. Gondor would be incredibly weak at this time. So it would not have the military power available to move north and impose its will in force. So Gondor would probably send more weapons and supplies to its proxies.
This is where I will see a major split between Gondor and Mirkwood. Gondor as stated above, would be more interested in a buffer zone. Mirkwood and the other elven realms to a degree would want regime change. So Gondor would support the regional autonomy movements. But the elves would support the Easterling equivalent of the Whites.
The Mirkwood proxies would suffer from two major problems. The first is their ambitions would conflict with the demands for regional independence from Gondor's proxies. This would put them in direct conflict with armed groups with more power than they otherwise would have due to outside help. Plus, concessions to these demands would allow for more popular support, which would be essential for getting recruits and food.

Deployment

Meanwhile, there is the leadership question for those fighting the civil war. For the independence groups, they would naturally have a some luck getting solid leadership within their movements. But coordination between them would be harder. Which means that creating a united front would be harder to form between them Gondor would probably step in to try and impose this. But their limited resource commitment would mean that their ability to impose a united front would be weaker during the critical period.
The Whites would be scattered across the entire border region. The Russian Whites existed in the age of wireless communications but still struggled to communicate. So the Easterling Whites would have much more difficulty. Add in the problem of exterior lines making transportation of supplies and reinforcements between them more difficult, and a united front is much more difficult to achieve. Then there is the fact that there is no clear idea as to who should lead. The old order is dead with no clear replacement. So the White forces attempting to impose a new united Rhun government would see conflict arise over who should take over. Their attempts to create a united front against the Blue Wizard would be even more difficult.

Battlefield

The attack on the Black Gate would have left Gondor with a very secure base to launch its invasion from. It is also closer to Minas Tirith and established supply lines. The area is closer to Gondor’s allies, who at this point would be motivated by the same realist logic to ally with Gondor to attack there. Offensive realism would predict that the collapse of a hegemon (Mordor in this case) would see a rush of neighboring powers seeking to exploit the power vacuum and take as much territory and resources as possible. This could work in Gondor’s favor by providing added muscle to crush a militarily powerful and hostile force and secure its northern borders. At the same time, should it delay, it might lose out in the scramble for territorial gain to other powers.
This area is mostly light or scattered woods. The area near Mordor’s northern mountain range is mostly desert due to volcanic pollutants. There is one large lake in this area with heavy woods along the north – but no major rivers, hills or mountains once you get past the Marshes of the Dead. Easterlings are also very militaristic to the point that everyone fight. Including women, a rare occurrence in Middle Earth. It seems that they have equal or better arms, armor and training than Gondor’s. Especially compared to the Southern Fiefs.

Conditions

The last major consideration for Gondor is 4 of the 7 dwarf clans live in the area in the Iron Hills. Guess what they mine. Also guess what is needed to further continue the expansion of Gondor’s military. Secure the territory that allows trade between Gondor and the Iron Hills and Aragorn secures needed supplies for his war machine. I suspect the dwarves would be interested in the contracts.
The more important issue facing Mirkwood's proxies is an issue that they would share with the Gondorian proxies. The Blue Wizard led "Bolsheviks" would control the central portion of Easterling territory. This means more people, economic resources, easier communication and shorter supply lines. Not to mention the legionary forts. The Easterling's larger cities would probably also be in this area too. Because their popular base is also united on helot grievances leading to a more unified political program, they would be much more united.
The independence movements would have these to a degree. But to a much more limited extent. First, their local territory would be much less. Which means fewer resources and potential recruits. They would also inevitably come into conflict with other tribes over disputed territories. Clashes between them would result in an attempt to settle borders. But the Whites would probably not have even this kind of territorial base.

The Engagement

How would Aragorn take on a well equipped and militaristic power to his northern border with little to no natural protection to exploit? Honestly I think he should move his troops all into Udun. I would probably send the remnants of the extra recruits from Denethor's professional force. This puts men in short striking distance to the Rhun.
Due to the need to consolidate his own internal power before a new major war is started, Aragorn would go attempt negotiations. Then he would call in the army to either defeat militarily those who resist or to take over occupation duties. Should these terms be rejected or the treaty broken, invasion and occupation should follow. Should either of these happen before the serfs are mobilized for the next campaign season, the goal should be to aim for a battle of annihilation. The idea is to so cripple the enemy forces in a single battle that they have no option but to seek unconditional surrender.
To do this successfully, I would think the best option would be to bait the Easterlings into attacking near the Dead Marshes. The combination of being near Rohan and thus its mounted troops, the fortifications of the Black Gates, the remains of the camp and the fact the battle can be watched from the air (from the mountains), it is possible to seek some encirclement of the army. When faced with a larger force, defeat in detail is a common tactic where you attempt to break the enemy into smaller portions that can be more easily defeated with your own forces. A famous example is Tannenburg. One of the factors that greatly helped the Germans there was aerial surveillance and marshes. Both Salamis and Thermopylae used forcing the enemy into a narrow pass that keeps a superior force from using their numbers is an effective tactic as well – something that could easily be done by Gondor as well.
The idea would be to get the Easterlings to send as many troops as possible south towards Gondor while keeping the main army near the Black Gates. Rangers would be used to keep an eye on enemy movements and kill stray detachments. These come from patrols or from being baited away to deal with the harassment. Either way, constant, low level attrition is the goal. Then, when the Easterlings get near the dead marshes, Rohirrim and tower guards are to be called up to block passageways so that the force can’t be re-consolidated after it is broken up to move through the hills and marshes. The main army is to then come out of Udun to pincer the Easterlings, cutting them off from their supplies, encircling them and moving in for kill.
Should this fail, there are many fall back positions that can allow Gondor to replicate the retreat and don’t die approach from Minas Tirith. At least two different fortified areas can be used for this purpose. But this would shift the battle into a more hit and run battle of attrition.
Once this battle is won, any further actions against the Easterlings can be conducted. I suspect that Aragorn’s impulse would be to move in to solidify control over the taken areas. At the same time, the Easterlings might take advantage of the truce and Aragorn’s attention being diverted south to attack then. This would then provoke a shift in troops back north. This could be done with relative ease at this point due to the larger forces Gondor has available now. The Easterlings are more urbanized than the Southrons, so Gondor is likely to have to conduct sieges to defeat the last bit of Easterling resistance.
Sieges at this point of history often followed a bit of a pattern. First the attacking army would arrive. Then a parley would occur – the two leaders would attempt to negotiate a surrender. This was often very tricky to pull off. Clearly the defenders did not want to give up their lands and homes, yet losing the siege could be more devastating. To make that point, the attackers would send an ultimatum after a failed parley offering to let those that surrender go with no harm to them. Those that didn’t and the city or castle fell would be butchered. The attackers justified this as a form of retaliation for the deaths of their friends they could have avoided if the defenders surrendered. It also sent the message to others in the attacker’s path that such threats are to be taken seriously. As a result, future sieges are more likely to end in surrender.
This pattern is likely to play out across the Rhun as Gondor’s armies approach, with the mostly recent and less well trained conscripts being more inclined to atrocities. Aragorn’s corruptible position is likely to become much more influenced by the cold, utilitarian logic that justifies atrocities out of a feeling that future crimes are being prevented. Add in the costs of keeping prisoners is high enough that Aragorn will be more tempted to resort to killing prisoners as the campaign continues.
Meanwhile the Easterlings tend to fight like that to begin with, so they might not be as shocked when they are on the receiving end of it. Population losses might not weaken over all military power due to the fact that fortune favors the defensive. This also means that the Easterlings are more likely to fight on, feeling that their civilization is user assault. The result is another prolonged and brutal campaign that becomes more and more savage over time. Aragorn’s actions will become more prone to massacres as the first option to crush a hardened enemy. The Easterlings then become more prone to resistance at all costs because they feel this is a war of genocide. The security dilemma sets in and people die.
Historically, populations have been taken by forces 1/6 their size before. This is not to say it would be easy, but possible. While the militarism and equipment would make them hard to subdue, this would be a problem that would have to be dealt with in the occupation phase. But much of this problem with occupation would be handled not by Gondor and the Lonely Mountain but their new client states. For now, the recent defeats and scattered population should make it hard to muster enough force to prevent conquest.
Keep in mind the Norman Conquest of 1066 saw at most 12,000 men taking a population of 1,710,000 people. Which means that a direct attack by Gondor could prove effective. Again, we need to remind ourselves of context. Their society has already lost what I estimate is a huge chunk of their population and getting the people they would need to compete in an open battle would be crippling. Lets assume that through deaths and men surrendering, the entire attacking Easterling force is lost. Because of holding back an Auxilia brigade for each fort, this translates to 365,760 lost. Granted, it is unlikely that all would be dead. But all would be wounded or sick from disease - major problems for armies in ancient times. Then, some would desert or get captured. All told, effectively those formations would be totally lost.
This represents between 4.6% and 7% of the entire pre war population. As noted elsewhere, these are the political elites. Not just the best troops, but also the ones most useful in leading and training a new army. They are also the ones holding together the society enough to resist aggression. Deaths are also almost a best case scenario. Wounded pose drains on the economy can can impoverish the rest of the community if in large enough numbers. Captured soldiers were ransomed if they were nobles. Otherwise prisoners were more likely to be executed due to the fact that armies at the time simply did not have the resources to feed prisoners and their own troops at the same time.

Outcome

Basically, we have a high likelihood of the Easterlings rallying around a new central government to deal with a host of new external threats. Gondor and the Lonely Mountain being the most direct. This would lead to massive losses of people and territory as the Easterlings would struggle against two professional armies after their own was wiped out. The end result here I expect would be a total conquest of the Rhun. Or the combined pressures of external pressures would be too great and the Blue Wizard would fall before he could consolidate his rule.
More likely a moderate outcome would occur. The Blue Wizard would be left in control of a damaged society desperate for peace and stability. A desire for revenge would likely fuel long term calls for war. But for the moment, the Blue Wizard would be forced to sue for peace at any price after a few early but irreversible defeats. This price would be a vast reduction in territory claimed/conquered by Gondor, the lonely mountain and allies. He would be rewarded for this with gratitude from a population wanting nothing more than normalcy a bit of stability and some food.
So, now lets look at what comes after. Remember that as Aragorn’s power grows, so too should his tendency towards evil and atrocity. At the same time, Aragorn has expanded Gondor’s reach into lands it has not occupied in thousands of years, and in some cases never actually held. Which means Aragorn has to contend with the pressures of occupation and colonial administration. Meanwhile, the Easterlings would probably have been seriously reduced in power due to their defeats. Their more centralized power structure means that there was a pre conquest threat much greater, but break that organization and all else falls. The result is that the conquest is harder but occupation easier. The good news is that of the more moderate peace option is what occurs, the harm to civilians would be less and Aragorn's pull to the dark side would be less severe.
For surrender terms I would propose disarmament, demobilization and resettlement to reduce the power of the military. I would also ask for reparations meant to make rearmament hard to accomplish, but not meant to be punishing. Mordor was the main enemy after all. Then a small amount of territory just north of the Black Gate should be surrendered as a means to setup a buffer area.
This buffer would ideally be up to the Sea of Rhun. This would allow for more farmland. There are rivers that flow into it that would be useful for trade. Then, this would constrain the available avenues for possible future invasions. Just as important is the historical reasons. Gondor's furthest reach into the area got to this same point. Securing here would go a long way to convincing people Aragorn is actually saving Gondor and starting a new golden age.
What this means is that conquest is likely inevitable and should the Easterlings decide to raise another army it would be devastating for them. This also means there is no counterweight to the Blue Wizard. They will take over effectively. There would be a patriotic rally around the Blue Wizard to help solidify his power. So I doubt a full toppling of the regime would happen now. That would depend on if the new regime is able to overcome its difficulties like the Soviet Union. Or it could fall like the Weimar Republic in the face of its problems.

Aftermath

The result is that of the areas that are conquered, the Rhun would be the first place to begin settling troops. Going with the 40 acre figure, the land grants would need to total about 394 sq. miles. Rhun – at least the parts near Gondor that it could conquer. That alone should provide enough space. I chose Rhun for the land grants mainly due to the fact that the major increase in food production is needed for either the military establishment located in Udun or for export to Moria – both are closer to Rhun than Harad. The devastated regions of Godor and Rohan are also the regions closest to Rhun as well.
It is unlikely this would continue on as a group of smallholders. Some are going to get worse land or be worse farmers, leading to the loss of their lands. Marriages will lead to some consolidation. Add in the need for manufacturing specialists in these new communities and some will choose to sell their land and go into another line of work.
Then there is the issues of who wins internally. I think a similar outcome to the Russian Civil War would occur. The Auxilia based white movements would probably all be defeated. Fearing reprisals, I think they would flee. But the atrocities committed by Easterling forces would make Rhovanion a poor choice of destination. They would likely be forced to move all the way across the Misty Mountains to Arnor. The Northmen would probably harass them the entire way. Their resulting settlements would get humanitarian aid from the Elves, probably mainly Rivendell. Moria would jump in with economic aid.
The Gondorian backed independence movements would see some successes. But some would be defeated too. Those that win would likely be areas Gondor would use to resettle refugees. It is doubtful this would end well. Ethnic tensions would naturally flair. The new state would feel its legitimacy is based on a degree of ethnic claims to the specific territory and the refugees would upset the demographics providing that legitimacy. But the economic strain on Gondor would also mean that Gondor would not want to keep everyone trying to come within its borders.
The Dwarves would take over directly the most economically useful territory on the border. Client states set up by dwarven colonists would probably form around borders with the Iron Hills. They would probably rely on military and financial backing of the Lonely Mountain to survive. But the area would see the rise of a dwarven elite ruling areas formerly part of Rhun's central government with Easterlings forming the lower class. Rule by an elite outsiders or foreigners is a recipe for race based conflicts in the future.