Sauron less powerfull then his old master Morgoth though still had his gratest ability of convincing. Not common to Morgoth he made a great ranking system which was a sign of order. Here you'll see how the ranking system of Mordor and generally Sauron's forces looked like?
1. General idea of system
Sauron's forces were mostly consisted of orcs though unlike the Morgoth's forces Sauron's orcs were divided in groups, clases and region communities. The large society is ruled by high ranks in each region. That allow Sauron to gain totall control over orc areas.
2. Orc races - understanding orcs
To understand hierarchy system, we must understand orc society. The orc species is notable for having difrent races who serve as casts.
The most nobile orcs are Boldogs which were ones Maiar who entered bodies of powerful orcs. These were powerfull and nobile breeds which served under command of Balrogs. In Sauron's armies These orcs were in charge of whole region armies and served under command of Nazghul.
Then we have normal warrior orcs. These are soldiers of the dark powers. They serve as army and can earn some small prices for fighting including plundering.
Other lesser orc breeds serve as hunters, trackers, scouts and informators.
Snaga is orc race which is treated almost as slaves. In fact in dark speach Snaga means a slave. They're ussually workers or emergency fighters. Have almost no position and can't be promoted in the ranks of Sauron.
Understanding primitive nature of orcs Sauron swiftly developed system that fits for their society.
3. Official armies of Mordor and Morgul
Besides many orcs which were living in the wild areas of Rhovanion Sauron's official force was actually in Mordor. Armies of Barad-Dur and Minas Morgul are largest armies situated in Mordor mainly meneged for assualting lands of Gondor. Those orc legions are formed from two diffrent groups:
Orc of the red eye - Sauron's personal forces. They are situated in Mordor parts of Gorgoroth and Udun. Some humans of Harad, Khand and Easterlings which inhabited those areas may also be a part of that force. These Mordor orc/human armies often march with official armies of Easterlings from Rhun. These orcs have a symbol of red eye of Sauron. They have black armor and chainmails and they're arramented with curved blades, huge bladed swords, poisoned knifes, short bows with black arrows and sharp spears.
Orcs of Morgul also known as orcs of half-moon are armies of Minas Morgul. They wear powerful armor with helmets and chainmails. And on their helmets and chestplates they have a symbol of half-moon. This force has more agressive role then Mordor/Barad-Dur armies which are protecting lands of Mordor. Morgul armies are assualting enemies of Sauron first.
Both of these groups are most influencial groups of warrior cast. They form most of the warrior race.
4. Wilderness areas
Wilderness areas of Rhovanion and Eriador are full of orc groups. This is a place were lesser orc breeds live. Especially in Mirkwood, Ettenmoors and Misty Mountains. There live orcs which are feral full of violence characteristics.
These orcs often tame huge amount of beast like wolfs, trolls and cave bats. Many times these orcs decided about fate of Middle-Earth like on fields of Gladden and under the Lonely Mountain. Those orcs live in small groups in caves or dark well hidden lairs, groups are divided under smaller areas and these areas units form whole region communities. This divisions can be summoned by the Dark Lord for important frontlines or. Though many of wilderness orc plunder nearby villages independently which is anyway advantage of Sauron.
5. Forming units
Orcs from both wilderness areas and Sauron's lands can form companies. Each company is commanded by a captain. Captains are warrior or lesser class orcs promoted to rank of captain. In Mordor companies are formed from both of orcs and men who inhabited in mordor. They were black ragged chainmails & armor and huge amount of diffrent weaponry. Company can beare at least one banner. Those companies can form divisions commanded by several captains. Divisions are able to have beast or war machine. Finally the divisions can form a legion. Whole legion is a combination of all orc units and it's commanded by a warchief(higher ranked captain).
6. Other species in armies of Sauron
Humans are also a huge part of Sauron's military. They were often assigned for campains on north and south. Most of Sauron's allies is from the south & east but here are also some from the north.
Easterlings are the most loyal to Sauron of all people. In the First age they served the fallen Valar and first Dark Lord, Morgoth. For Sauron Easterlings are much more valuable then orcs.
The advantage of those people is that they do not fear fel and darkness. Great example of this is Warlord Gothmog who's Easterling commanding forces of most devious fortress, Minas Morgul. Easterlings live in tribes which serve asdiffrent units. Balchoth and Jangovar serve as unbreakable infantry and wainraiders are serving as cavalery. Even after fall of Sauron they will still revere their Lords, Sauron and Morgoth.
Haradrim are people living on the south. They're revering Sauron as their god. They also have great relations with Easterlings. Haradrim infantry march on feet, the cavalery ride horses and tamers are mounting mumakil. Mumakil are covered by chainmail and mounted by Haradrim on the towers which are on backs of each Mumakil. Both well organised warriors of Near Harad and feral vetrans of Far Harad are willing to kill or die for Sauron.
Variags of Khand are impetuous people living near Mordor on the lands of Khand. Variags are considered to once be Easterling tribe so they have good bounds with dark powers. Variags often march with Haradrim against the inhabitants og Gondor.
Corsairs of Umbar are tribe which was formed in the port of Umbar by Gondorian rebels, Black Numenorians and Haradrim. Because mostly formed by Haradrim corsairs have great relations with Harad. Powerfull and resistant warships carry lots of corsairs and haradrim warriors. They're raised by slaves and inormous sails. Black wooden ships and dark or red sails with symbol of red eye or eal means that you should run as fast as posible.
Black Numenorians were once honorouble people from Numenor. But after fall of Numenor those who remained in the south bounded their lives to the dark powers. They've became powerfull sorcerers, soothsayers and necromancers. They're most trusted Saurons servants. After rebelion in Umbar most of them mixed their blood with foremer Gondorians and Haradrim forming the brood of Corsairs from Umbar. Some of them though remained uncontaminated by other broods' blood. Those served Sauron as powerful commanders.
Feral people living in the land of Dunland. They had many conflicts with Rohirrim. They allied with Sauman(Sauron's allie) during the War of the Ring. Arramented with axes, maces, spear and shields they marched with Isengard army to Helm's Deep.
Angmarians are people of north kingdom Angmar. Their lord Witchking of Angmar led them against Arnor in their war. Angmar had powerful soldiers both infantry and cavalery. They've marched with banner of their Nazghul Lord and a banner of the Red Eye.
Ettenmoors tribes are feral men who marched with armies of Angmar during War of Arnor. After fall of Angmar almost none were left from this impetuous brood.
Rhudaur warriors were once Arnorans but during invasion of Angmar their king betreyed rest of Arnor alling with the Witchking of Angmar.
Inferior dwarves are those dwarves who were banished from their clans. Those dwarves allied with Sauron in the Second age to improve their lifestyle.
Dwarves from clans living on the east were allied with Easterlings and the Dark Lord. It is possible that they took part in attacking Erebor with Jangovar Easterling tribe.
All of those people were in the service of Sauron.
7. Other orc lands
Dol Guldur is the dark Necromancy fortress in the south mirkwood. It's under command of Nazghul, Khamul. Orc army from those areas often harrased Sylvan Kingdom and Lothlorien. It's inhabitant also often allied with Easterlings.
Isengard during the war of the ring was allie of Sauron. It's overseer Saruman created Isengard Uruk-hai and allied with Dunlendings harrasing lands of Rohan.
Moria was once inhabited by dwarves but when they diged the Balrog it forced them to leave their homeworld. After that Sauron contacted the Balrog and made a primitive "alliance" with the Balrog. After that Dark Lord sent his orcs to Moria to inhabit those areas.
Gundabad mountain was a powerful orc fortress. Orcs from that location were strong and had a great connection with Dark Powers.
8. Who's in charge?
Sauron had many powerfull assets from necromancers to wraiths of dead.
The most powerfull were Nazghul, nine wraiths. They were given controll over diffrent areas.
Also Black Numenorians were trusted sorcerers. One of them was Mouth of Sauron who served as luitenant of Barad-Dur. Sauron also allied with two maiar, Saruman the Istari and Balrog the Durin's Bane. On the last highest ranks were influencial humans like Gothmog and nobilitive Boldogs like Azog and Bolg.
9. Assigning under commands
Each orc has his Nazghul/overseer number. Those numbers are assignig orcs to the regions.
For example Minas Morgul - 2, Mordor - 1, Dol Guldur - 3 e.t.c.
Mouth of Sauron - Witchking of Angmar - Khamul
Durin's Bane - Other Nazghul - Saruman
Warlords: Great Goblin - Golfimbul - Gothmog - Azog - Bolg - black numenorians
orc captains and warchiefs
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men & dwarves orc warriors
orc warriors of the red eye and half moon
Dol Guldur and Gundabad warriors
Moria and Isengard Warriors
lesser orc breeds (hunters, trackers, informators)
I hope this post helped you understanding military of Sauron and his great masterminding.
Let me know your thoughts bellow in the comments.