To assess the military strength of each force, you have to consider their assets (personal strength of the individual is considered in the follow up section)
Voldemort's War Assets
- A relatively small cadre of Death Eaters, I'm willing to guesstimate the total at 50. The powers of the average Death Eater includes:
- Use of the Killing Curse, effective auto-kill against all living beings. May or may not be effective against Sauron or undead entities such as Nazgul (considered later)
- Access to unlimited broom flight, duration limited only by exhaustion, height limited by atmospheric pressure, and only then in theory
- Access to instant teleportation through Apparition
- The full host of commonly known HP Universe spells, both defensive and offensive, as well as a smattering of Dark Arts spells. Expected to be at least competent or exceptionally competent Wizards/Witches.
Sauron's War Assets
- A massive army of Orcs, of various breeds and capabilities, estimates hinting towards as many as a million. Capabilities and limitations include:
- Standard melee combat
- Hardy nature and survivability
- Mediocre archery skills
- Some breeds have a weakness to sunlight
- A significant quantity of Trolls, numbers not given. I'm willing to estimate somewhere in the hundreds, perhaps a thousand. Capabilities include:
- Extreme brute strength
- Highly durable
- Weakness to sunlight
- A not-insignificant quantity of Southron and Easterling Humans, numbered in the thousands. Capabilities include:
- Average physical and mental powers of a lower class human warrior
- Comes with a free fleet of ships!
- Oliphaunt mount capabilities
- Average archery skills
- 9 Nazgul, with Fell Beast flying mounts of unknown quantity. Capabilities include:
- Effectively invulnerable due to Wraith curse. Killing them results in a "down-time" period as their soul returns to Sauron to be restored. Distance from Sauron has an impact on this time . . . books imply this can be as long as several weeks.
- Capable riders both on horseback and fell beast, the latter of which offers flight
- Possess a crippling fear scream, capable of breaking morale. Can be counteracted by allied morale boosters.
So the stage is set; Voldemort and his 50-some HP Universe Death Eaters versus an army numbering in the hundreds of thousands. Sounds like a pretty easy win for Sauron, right? I beg to differ.
Despite the massive quantities of humans, orcs, and trolls, even a thousand of them aren't worth a single HP universe Death Eater. Every one of those targets could be instantly eliminated with a cast of the Killing Curse, or killed in more numerous quantities with potent spells like Fiendfyre. With their maximum combat range limited to archery distance, the Death Eater squad could simply hover out of range on their brooms and cast KCs every few seconds for hours on end, slowly whittling down Sauron's numbers with limited or no losses on their side. Assuming each Death Eater could kill 10 Sauron allied units every minute, a team of 50 Death Eaters could eliminate 5000 enemy units in 10 hours, the whole time floating safely out of archery range and unleashing rangeless KCs. AoE spells like Fiendfyre would only amplify this. Even if they came into archery range, Death Eater use of the Protego charm could protect against arrows within reason, so even minor slip-ups would not be an issue. Even Trolls, being a living thing, are susceptible to the insta-kill of KC, so their ground based brute strength is irrelevant.
After 10 combat hours, the Death Eaters could fly or apparate away to a safe location for rest, recovery, and preparation for the next assault.
Nazgul aside, this sets up the Death Eaters in a scenario of excruciatingly slow but brutally one-sided combat. With 5000+ enemy casualties a day, this would lead to an effective 50% destruction of Sauron's forces within 100 days. It is my opinion that Sauron would take strategic measures to circumvent this, because despite having a cannon-fodder oriented style, there is no point in literally throwing your forces away. The only reasonable strategy he has against the Death Eaters is that of the Nazgul.
The 9 Nazgul are the only force that could give the Death Eaters any sort of opposition. Using their fell beast mounts, they could assault the Death Eaters even in the safety of the skies. However, based on the top speed depicted in the films by both fell beasts and broomsticks, I do not believe that the Fell Beasts could ever catch a Death Eater in chase. Furthermore, Fell Beasts are living beings, and can be killed by the Killing Curse, meaning that the otherwise melee-range Fell Beast/Nazgul combo would likely be beaten before reaching the Death Eaters. However, it is possible that with sufficient stealth, an ambush could be set up which might allow the occasional Death Eater to be killed.
The Nazgul, on the other hand, are an unexplored territory when it comes to the Killing Curse. Being undead, they may or may not be hindered by it, and even if they are, it may just reset them to wraith form . . . this is not a permanent solution. Until such time as a permanent kill technique can be developed, the Nazgul will be a frequently returning menace that will harass the Death Eater forces time and time again. However, there is no quantification of how many Fell Beast mounts they had in reserve, so this is a limiting factor.
A more important factor would be the morale-crushing effects of the Nazgul scream . . . depicted in the books, it is capable of turning the tides of battle by terrifying living beings through supernatural power. This could be best put to use by rapidly using the full 9's screams in unison to shock and awe the Death Eaters, possibly putting them off guard long enough to eliminate some of their number.
Given that the DE assault on Sauron's forces would be a slow battle of attrition, the success of this technique would determine the outcome of the war; would the Nazgul scream be potent enough to eliminate Death Eaters, and would the Death Eaters be able to learn from this in future encounters and develop a counter-strategy?
As depicted in the books, the presence of morale inspiring individuals or actions can nullify or even reverse the morale-effects of the Nazgul scream. While I'm not willing to place Voldemort on the field due to his nature as general, the presence of influential and strong combatants such as Lucius Malfoy, Bellatrix Lestrange, or Fenrir Greyback could counter-act this to a degree, and particularly stunning actions on their part (Fenrir jumping onto a Nazgul and decapitating its Fell Beast, for example) could provide a morale boost that actually repels the Nazgul.
So for all of Sauron's countless horde, the true battle comes down to the following:
Can 50 Death Eaters slowly, slowly whittle away at Sauron's forces over the course about 6 months to 1 year without taking substantial losses at the hands of the respawning Nazgul (who may show up as often as every few days, or as rarely as every few weeks). The end result would probably be determined solely based on how many Fell Beast mounts the Nazgul possess, and the effectiveness of the Nazgul scream over the course of dozens of attempts on the Death Eater forces.
Conclusion
My vote is that the Death Eaters may take a few casualties the first few times to the Nazgul, but would adapt their tactics shortly after to eliminate the Fell Beasts from the air upon each assault, learning to deal with the fear effect as they gained practice. After that, it is simply a matter of staying smart and not screwing up as they slowly mop up Sauron's armies until there is little left but stragglers and deserters.
But what about Sauron himself? Well, see below.