Avatar: The Last Airbender's world features four great nations based on the classical Greek elements, with the industrious Fire Nation looking to the future and the bureaucratic Earth Kingdom favoring stability and tradition above all. Meanwhile, the Water Tribe is a spiritual and reclusive place split into Northern and Southern regions.
However, while the Northern Water Tribe has always been powerful and well-defended, like a miniature Ba Sing Se, the Southern Water Tribe's natives have known nothing but hardship and strife for decades, both in Avatar Aang's time and Avatar Korra's time. In fact, the Southern tribe and its unique customs came close to being erased from the world forever.
The Southern Water Tribe's Struggle Against The Fire Nation
In the past, both halves of the Water Tribe were roughly equal in population, wealth and military readiness, but the Hundred-Year War changed that. While the Northern Water Tribe held firm against the Fire Nation's fleets, the Southern Tribe suffered repeated losses, especially against the Southern Raiders.
The Fire Nation kept raiding the South over the years, capturing more and more of its Waterbenders until the last one, Hama, was also taken. The Southern Tribe was thus pacified, but was still fortunate that the Fire Nation didn't see fit to wipe it out entirely. A generation or two later though, Katara's mother Kya kept hope alive by claiming to be a Waterbender, and thus died in Katara's place.
Katara alone embodied the Southern Tribe's unique waterbending style. When Avatar Aang was released from his iceberg shell, Katara taught him all she knew about waterbending while journeying to the Northern Water Tribe. However, this left the Southern Tribe defenseless -- not even their non-bender warriors remained as a garrison force. Hakoda, chief of the South, was busy raiding the Fire Nation with his own miniature fleet, which was the Southern Tribe's only real defense against their adversaries.
Fortunately, the Fire Nation was too preoccupied with the Earth Kingdom campaign to bother destroying the South, and when the war ended, the Northern Water Tribe finally lent aid to their Southern neighbors. As Avatar: The Last Airbender's Dark Horse comics showed, the Southern Water Tribe soon rebuilt its infrastructure and its population swelled due to immigration, while factories were set up as well. The South was finally back in business.
The South & North's Strained Relationship During Korra's Time
Some seven decades later, the South is in trouble once again, but not because of the Fire Nation. Unalaq, the chief of the North, laments the lack of spirituality in the South and says as much to his niece, Avatar Korra. However, Unalaq's plans for the South are not so benign -- he is hatching a plan to reconnect the spirit world with the mortal realm by freeing the dark spirit Vaatu to restore balance (or so he thinks).
The Southern Water Tribe members want nothing to do with this, so Unalaq forces the issue. He sends an occupation fleet to seize control of the South and spite his brother, Tonraq (Korra's father). Tensions are high, and things only get worse when some Southerners try to kidnap Unalaq in the dead of night.
Korra, as both the Avatar and a Water Tribe native, attempts to broker a truce by compromising on Tonraq's fate (he was framed for kidnapping Unalaq). However, Korra learns that her father is innocent and frees him, and civil war erupts. The Southern Tribe may have recovered from its low point during the Hundred-Year War, but it's still no match for the powerful North, and Tonraq can't hope to win this civil war. Worse yet, the innovative Varrick is fueling the war effort for profit, and Avatar's world is nearly plunged into one hundred centuries of darkness when the Southern spirit portal is opened and Vaatu is freed.
The South becomes a fierce battleground for the world's future, along with the distant Republic City, until Korra finally triumphs and seals Vaatu again. Peace has returned to the Southern Water Tribe, but despite its remote location, this region is never far away from brutal conflict.
