The War of Worlds, also known as the Troodenite War, was a major galactic conflict that occurred 53 to 57 years after the Battle of Yavin.
A long time ago in a galaxy far, far away....
After 100,000 years of their ancestors being in isolation on the planet of Trooden, in the Unknown Regions, the Troodenites were desperate to gain territory in the other unknown regions of the galaxy, namely Rakata Prime (also known as Lehon), which was Trooden's closest neighboring planet. By 50 ABY, Rakata Prime was already a barren wasteland with polluted oceans and surrounded with asteroid-like rings of rubble (mostly scattered remains of the ancient Star Forge that was destroyed in 3,956 BBY during the Battle of Rakata Prime) from wars previously fought on the planet. The bad condition of Rakata Prime was not appealing to the Troodenites and their Emperor. The Troodenite Empire itself was a product of many tribal wars that have taken place on Trooden throughout the ages. Ethnic Troodenites constantly fought with a race that they themselves may have evolved from, the Saronians. As soon as the Troodenite tribes won the war against the Saronians, they adopted much of the Saronian culture and government, which was aristocratic in nature. They also gained access to Saronian legends about Troodenite/Saronian origins and hidden knowledge about how to control the Star Forge that were recorded in documents written in Saronian hieroglyphics, which only few could read. The Saronian documents also held many secrets to an ancient past in what is now known as the Core Worlds. The Saronians called it the "Near Middle World"; this possibly refers to the planet of Duro in the Core Worlds, because both Saronians and Troodenites physically resemble the Duros). Now that the Empire gained access to the knowledge about the Core Worlds and a planet of origin, their next desperate attempt would be to find it and rule it, along with other territories gained through war.
The Saronians were the descendants of Duros slaves transported to Trooden by Rakata slave traders before the rise of the Infinite Empire 70,000 years later (35,000 BBY). After the age of Rakatan slavery had ended in 25,200 BBY, the Saronians had already inherited much of Rakatan knowledge and culture. They gradual became a thriving society that developed technologies married to the Force, another Rakatan influence. Tribal wars ensued between Saronian tribes because of thirst for power and territory among their leaders. One tribe called Tirudān, named after one of its chiefs, had been driven out of Saron (a tribal territory on Trooden) by the more powerful Saronian elite, the Aristocracy. The Tirudān would eventually evolve into a species separate from the Saronians and become a nation more powerful than the Aristocracy, later becoming the Tirudān (Troodenite) Empire.
The War of Worlds
53 years after the Battle of Yavin, a devastating blow to the Galactic Alliance by a new adversary, from a planet in the Unknown Regions, had taken place. The Troodenite Empire began to invade the known galaxy by first overthrowing the Monarchy of Gonstar on the planets of Mutu Goūna and Mutu Ora within the Gonstar Sector of the Inner Rim. Gradually making their move toward Coruscant without being detected by the Alliance, whose control was predominately within the Core Worlds at the time, the Troodenites took over many planets in the process. These planets were not under the control by the Alliance, nor were they under the control of the Fel Empire, whose control was predominately within the Imperial Space of the Outer Rim. The Alliance and the Fels were the two dominant powers in the galaxy at the time of the invasion. But the Troodenite presence wouldn't go undetected very long after the invasion hit Coruscant in 53.9 ABY. As soon as the invading presence was detected at Coruscant by the Alliance, the Chief of State Divilan Sivilain called a meeting with Dag Tchilat, a Commander of the Yuuzhan Vong warrior caste, and Kenth Hamner, Grand Master of the New Jedi Order, on the planet of Denon. There, they made a pact that was known as the Triple Agreement, a pact that would reconcile three galactic powers that were enemies of each other in the past; the Galactic Federation of Free Alliances, the Yuuzhan Vong Empire, and the Jedi Knights. These three powers became known as the Tripartite Powers.
By then, the invading empire had gained much territory within the Inner (I.R) and Outer Rims (O.R.) including the Gonstar Sector (I.R.), Kiffu/Kiffex (I.R.), Rattatak (O.R.), Firrerre Sector (O.R.), and Zonju V (O.R.). The Troodenite Emperor Shahltoon had ordered his armies beforehand to enslave the inhabitants of whatever planets they takeover, and force those inhabitants to fight for the Empire. The Fel Empire at that time had several Moffs that "held a distaste for the Galactic Alliance, leading to personal Cold War-esque hatred." The Alliance could not have had a trust worthy relationship with the Fels, therefore, they did not make a pact with them to defend the galaxy against the Troodenites. One possible reason for the Fels' resentment toward the Alliance is that its Supreme Commander was a female, Natasi Daala; the Moff Council, at that time, still had male chauvinistic prejudices against women.
In times of desparation, Chief of State Sivilain and Supreme Commander Daala had no choice but to form an alliance with two past enemies of the Galactic Alliance. When the Vong and the Jedi Order agreed to be part of the Triple Agreement (after much quarreling, of course) on Denon, they made their first move against the Troodenites at the Battle of Coruscant. To the Alliance's dismay, the Troodenites also made a pact while establishing command posts on the planet of Empress Teta, in 53.8 ABY. The Troodenite colonists encountered a remnant of an ancient Sith-society that was known as "Krath", dating back to 3,997 BBY. These remnants called themselves "Jen", which in Sith language (and in their context) means "hidden". The Jen are most likely the Krath cult that Sebban Keto had established in Cinnagar sometime during the Rise of the Empire Era. The pact that the colonists made with the Jen was known as the Treaty of Cinnagar. This treaty ensured that when/if the Empire wins the war against the Galactic Alliance, with the help of the Jen, then Emperor Shahltoon will be gracious enough to let the Jen have a "cut" in governance within the "soon-to-be" Galactic Empire of the Troodenites. This meant that the Empire would remove their command posts from Empress Teta, and let the Jen govern that planet as part of their own confederation, along with other near-by planets, including Coruscant. After a calming of hostilities between the Jen and the invading Empire, the Jen agreed to the pact, and by doing so, they gained the utmost respect from the Emperor. The Troodenite Empire and the Jen Sith Confederacy together became known as the Core Powers. As a token of his gratitude, the Emperor gave the new Sith Confederacy the planet of Khomm, in the Deep Core region, as a token of his gratitude. The colonists captured the natives of Khomm and gave them over to the will of the newly established confederacy; this was known as Troodenite-Jen Slave trade. The Khommite captives were then forced to fight in the battle that sparked the start of the War of Worlds, the Battle of Coruscant after the Troodenite invasion.
The Battle of Coruscant
Just before the time of the first great battle of the War of Worlds, the Battle of Coruscant, Alliance Chief of State Divilan Sivilain had not formally appointed a Supreme Commander to head the Galactic Alliance Defence Force against the Troodenites. The Alliance had not had a Defence Force Commander since Gilad Pellaeon and Cha Niathal in the early 40's ABY, so Sivilain appointed former Chief of State Natasi Daala to be Supreme Commander over the Defence Force. In turn, Daala would have to sacrifice her anti-Jedi sentiments and let Grand Master Kenth Hamner take the role of High Jedi General of the Defence Force, making him the first Jedi ever to serve as Army Commander for the Galactic Alliance. At the behest of Hamner, Ben Skywalker was permitted by Daala to serve under the newly appointed Army Commander. Ben's father, Grand Master-in-Exile Luke Skywalker was not in the position to serve at that time because Daala still had distrust concerning him, but Ben had his father's blessing at Hamner's request, via information stored within the memory of a R2-series astromech droid named R2-E1 (Toowee). As soon as Skywalker got the informtion, he made out, upon the Jade Shadow, for Denon where he met with Hamner to prepare for the attack on Coruscant, leaving his father behind on the Sinkhole Station within The Maw cluster.
The Tripartite Powers commenced their attack on the Troodenite-occupied Coruscant by first sending in 15 coralskippers to make an assault on the Palace District, where the Troodenites had been residing at the time. The Troodenites were at first surprised because they didn't know where the missiles sent by the coralskippers had come from, for they were firing from above Coruscant's atmosphere. These were actually warning shots to notify that an attack was being commenced, and not aimed at killing anyone; it was meant to call the Troodenites out to space and keep any innocent civilians out of the fight.
After the missiles from "rock spitters" (plasma projectors at the front of coralskippers) hit the sides of the main palaces, it caused massive quakes to the buildings and grabbed the attention of the Troodenite occupants.
Behind the scenes
The War of Worlds was partly inspired by World War I of Real-world history. The "Triple Agreement" was inspired by the real-world Triple Entente made by the United Kingdom, the French Third Republic, and the Russian Empire. "Entente" literally means "agreement" in French.