Southron was a broad term for the Men inhabiting the lands south of Gondor. While generally a reference towards the Haradrim; it also reffered to the southern Black Númenóreans, the Corsairs of Umbar, the Troll-men and the "Southrons in Scarlet". During the Second and Third Ages, most (if not all) of these barbarians and renegades sided with Sauron in his wars with the Free Peoples of Middle-Earth.
During the Second Age, the barren desserts and dense jungles of Harad were occupied by a dark skinned breed of Men called the Haradrim. In SA 2280, an advanced race of island men known as Númenóreans settled on the coast of Harad, were they built a great city-state called Umbar. From here they become great allies and teachers to the Haradrim, but over the years, Sauron corrupted the Numennoreans, and they began to conquer and enslave the Haradrim and other "lesser men". After the downfall of Númenor, most of those that survived became known as "Black Numenoreans" and founded 2 other kingdoms in Middle-Earth. The rest became known as the "Dúnedain" and founded the northern kingdoms of Arnor and Gondor, where they remained faithful allies of the Elves and other Free Peoples.
Around SA 2251, Sauron gave 9 Rings of Power to the 9 kings of the Easterlings, the Haradrim and the Black Númenóreans. At first, they profited from these gifts, using them to become great sorcerers and expand their kingdoms. But they quickly became corrupted by them, turning into wraiths with no free will. With their kings enslaved, the men of the East and South were forced to fight for Sauron in the War of the Last Alliance, in which many of them died. After the disappearance of Sauron and the Ring Wraiths, the kingdoms they ruled dissolved into chaos, with the exception of Umbar.
As the Third Age progressed, the Haradrim grew more numerous, and came to envy and hate the prosperous northern kingdom of Gondor (whose inhabitants had abandoned and ignored them ever since the War). They began to forge truces and alliances with their old enemies (the Black Númenóreans, the Easterlings and the Variags of Khand) and took to pillaging and raiding the southern border of Gondor. Many of their tribes were still ruled by Black Númenórean lords, and virtually all the tribes of Harad operated under the jurisdiction of Umbar.
The Black Númenóreans continued to occupy Umbar, from where they held great sway over the Haradrim. But that changed in TA 933 when Gondor captured Umbar in a surprise attack. The Black Númenóreans fled to their Haradrim subjects, with whom they unsucsessfully tried many times to reclaim Umbar.
After the Kin-strife of TA 1432, a devastating civil war between the inhabitants of Gondor, the defeated rebels fled southward to Umbar, taking with them a sizable portion of Gondor's navy. These outcasts became the Corsairs of Umbar, and drew to their number many brigands from the outlying settlements surrounding the Anduin. They took to pillaging the southern ports of Pelargir and Lebennin, intercepting trade vessels and abducting women to bolster their their community. This resulted in a series of conflicts known as the Corsair Wars. These wars ended in TA 1810, when Gondor reclaimed Umbar. The surviving Corsairs probably joined forces with the Black Numenoreans, and helped them stir up the Haradrim against Gondor and Umbar. Shortly after in TA 1846, the Haradrim (most likely those under the influence of the Black Númenóreans and the Corsairs) captured Umbar. After this, the Corsairs mixed with the Haradrim and the Black Númenóreans. They failed to regain their former control over Harad, however.
Many of the tribes of Near Harad sided with the Variags and the Wainriders (a confederation of Easterling tribes) in their war with Gondor. While the Wainriders and Variags attacked from the north, the Haradrim attacked from the south. However; they were intercepted and defeated by Earnil's smaller army, those that survived fleeing back to the South.
During the War of the Ring, the Corsairs and the Haradrim put aside their differences to side with Mordor against Gondor. All of the tribes of Harad were united under the leadership of a warlord known as the Black Serpent. By this time, 2 new groups of Southrons had come into existence; the Troll-men and the "Southrons in Scarlet". Little is known about either of them. The Troll-men came from the jungles of Far Harad, and bore a strong resembelence to Half-trolls. Nothing is known about the Southrons in Scarlet, other then the fact that they fought in the Battle of the Pelennor Fields.
Thousands of Haradrim went north to Mordor, while the Corsairs raided the southern ports of Gondor to keep reinforcements away from Minas Tirith. A good 1,000 Haradrim were ambushed and killed in Ithilien by Faramir and his Rangers, but at least 10,000 of them made it safely to Mordor.
BATTLE OF THE PELENNOR FIELDS
During the Battle of Osgiliath, an army of 18,000 Haradrim (and an unknown number of Troll-men and Southrons in Scarlet) came in from the south and helped the army of Minas Morgul defeat the Gondorians stationed there. The Haradrim then went forth with the armies of Mordor to lay siege to the citadel. The Troll-men and Southrons in Scarlet remained in reserve at Osgiliath with the Easterlings and the Variags. Meanwhile; the Corsairs deployed a fleet of "50 Great Ships and smaller vessels beyond count" manned by about 10,000 men, which moved up the Anduin towards Lebenin and Pelargir.
The Southrons would have won the day if it weren't for the arrival of Theoden and his army of Rohirim. Arriving on the Pelennor Fields by use of a secret path through the woods revealed to them by the Druedain, the 6,000 riders of Rohan charged the Orc ranks, routing the army of the Black Gate. The Haradrim retaliated with their Mumakil (giant elephantine beasts of Far Harad) and wrecked much destruction on the Rohirim ranks. However; the Rohirim overpowered the inferior cavalry and infantry of Harad and Morgul. Meanwhile; Aragon , the Grey Company and the Army of the Dead intercepted the Corsair fleet and slaughtered them. The South Gondor Army then took the Corsair ships and sailed them northward to Minas Tirith.
On the Pelennor fields; the Witch-King of Angmar, commander of Sauron's armies, was slain by a shield maiden named Eowyn. But Gothmog (Leutenent of Morgul) took command and sent the Easterlings, Variags, Troll-men and Southrons in Scarlet into the fray. Fortunately, the fleet of Umbar arrived, flying the banner of the king of Gondor. The Orcs quickly disbanded, but the Easterlings and Haradrim fought to the last man (it is unknown weather any of the Troll-men or Southrons in Scarlet survived).
BATTLE OF THE BLACK GATE
At least 10,000 Haradrim fought against the armies of the West in the Battle of the Black Gate. About half of them were killed, a quarter captured, and the rest fled to Harad.
After the War, Aragorn held a peace meeting with the leaders of Harad. Thankfully a truce was forged, and the Haradrim even gave him a mumak as a peace offering. The Corsairs were not so lucky. Aragorn denied their sovereignty, and forbade the Haradrim from assisting them. Most of the Corsairs fled Umbar. Those that stayed and fought were killed and captured in the battle. The Corsairs who escaped probably joined up with the remaining Black Númenóreans and those of the Haradrim who remained hostile. These Southron raiders probably pestered Gondor and Harad for a while, but dwindled over the years.
The Haradrim were most numerous and dangerous of the Southrons. There were 2 main groups; the brown skinned dessert nomads of Near Harad, and the black skinned jungle imperials of Far Harad. Ever since the end of the Second Age, the Near Haradrim lived in many seperate tribes, often at war with each other. They wore armor made of bamboo, wicker, bone, ivory, stone, bits of gold and jewelry. Bamboo and stone were also used in the manufacturing of their weapons, spears, shields, bows and arrows. Their metal working technology was virtually nonexistencent, but due to their connections with the Corsairs, you would occasionally found swordsmen in their ranks. Some of the wealthier tribes of Near Harad acquierd a lot of horses, weapons and armor through trade with Rhun and Khand. But by far greatest military advantage in the south was held by the Far Haradrim. They had somehow captured and tamed the giganitic mumakil. They drove them into the enemy ranks with spikes on their tusks, and on their backs were bamboo structures covered with archers and spear-men. By the time of the War of the Ring, a Harad chieftan called the Black Serpent had united all of the Haradrim under his banner. The army of Harad was now organized, comprised of swordsmen, spearmen, archers, horsemen and mumakil riders.
While not all of the Black Númenóreans were Southrons, most of them were. After the downfall of Numenor, the surviving King's Men fled to Umbar, from where they conquered and enslaved other men and established many colonies along the coast of Middle-Earth. They became bitter enemies of the Dunedain (their kin and descendents of the Faithful), the Elves and other Free Peoples.
Prior to the War of the Last Alliance, the Black Númenórean Kings, along with six others of Easterling and Haradrim descent, were transformed into wraiths by magic rings given to them by Sauron. They led their armies to aid him in the War of the Last Alliance. After losing this war, all of their colonies and strongholds fell into disarray, except for Umbar. Many of them went north to Angmar, and fought in the War under the command of the Witch King. Most them died in this war, with some fleeing to join the others in Khand and Harad. The Black Númenórean aristocracy continued to survive in Umbar until it was captured by Gondor. Most of the Black Númenóreans returned to Umbar when it was recaptured by the Corsairs, but some remained in the desert with their Southron allies.
During the War of the Ring, most of the Black Númenóreans (including one of their leaders, the Mouth of Sauron), went to Mordor, particularly the fortress city of Minas Morgul, where their knights were probably used to form the Morgul Cavalry. After the War, Mordor was conquered, Umbar was once again reclaimed by Gondor, and the remaining Black Númenóreans passed out of Gondor's knowledge.
The Corsairs were descended from the Dunedain rebels who were driven out of Gondor after the Kin Strife. They took Umbar as a base, and drew many outlaws and brigands from the river settlements into their ranks. They were driven out of Umbar by the Gondorians, but quickly reclaimed it with the help of the Haradrim and the Black Númenóreans. After this, the Corsairs interbred with their allies, becoming a mixed breed. The Corsairs were Gondor's most frequent enemies until after the War of the Ring, when they were driven out of Umbar once and for all.
The Troll-men of Far Harad were a little known of race. It is unknown whether or not they were actually hybrids of Trolls, Men and/or Orcs, or if they were simply an inhanced form of Men, Orcs, or mixtures thereof. They may been an offshoot of Men like the Hobbits, or simply a distinct breed of Southron Men.
They resembled the Half-trolls, with black skin, white eyes and red tongues. They, along with the other Southrons, joined forces with Mordor during the War of the Ring. They fought in the Battle of the Pelennor Fields, serving as reinforcements to the main army. It is unknown what happened to them after the War.
"Southrons in Scarlet"
Nothing is known about the Southrons in Scarlet other the the fact that they fought in the Battle of the Pelennor Fields, serving as reinforcements to Sauron's army. They may have been a distinct race of men or creatures like the Troll-men, or a multi-racial group like the Corsairs. It is more likely that they were a Haradrim tribe or group of tribes, or possibly a synonym for Haradrim.