In the ancient days of the Old Republic, Thule was inhabited by a Sith stealth regiment. They set up on Thule, masked by its remote location and the overwhelming power of the dark side. After the Old Republic repelled the Sith incursion about five thousand years vefore the Battle of Yavin, a small number of Sith devotees and dark side adherents retreated to Thule to regroup alongside the Sith special forces. They lost the fight but expected the defeat to be temporary. Together, they set themselves up as an advance force for the time when the Sith would return and have revenge.
Millennia passed, and no revenge was in sight. Sith loyalists occasionally remained in touch with forces in Thule, and generations of soldiers were kept sharp by the power of hate. During this time, the planet´s population dropped preciptously. At its low point, Thule had a population of roughly half a million people. The renaining die-hards were loyal but disgruntled with their misuse.
When Darth Bane reduced the number of Sith Lords to two, Thule swore allegiance to the lords who remained in the galaxy, believing that they would give the armies and students of the dark side a purpose again. Over the next thousand years, the Sith masters and apprentices did indeed use the resources on Thule for various schemes. Human troopers were siphoned off in small groups for various purposes.
Roughly a hundred years before the ascendance of Emporer Palpatine, dark side prophets at the Sith temple began receiving violent prophecies if Thule´s place in the Empire to come. The prophets saw that millions of soldiers would decend on Thule to help the Sith regain their place of honor and power. Just before the Clone Wars, as foretold, millions of Separatists came to Thule under the direction of Count Dooku. Equipment and troops from the Techno Unsion also began appearing in the skies, and the planet was abuzz with activity and preparation. Count Dooku held pieces of an ancient Sith weapon known as the Dark Reaper, and they were assembling it for use in the coming wars. The Dark Reaper was demolished when part of the wars came to Thuke, destroying the weapon before it could be properly deployed.
This was horribly demoralizing for the soldiers, who felt their destiny was thwarted. Thule did provide a staging ground for souldiers who fought in other parts of the galaxy during the Clone Wars, but it never experienced the glory its leaders wanted. Afterward, upon Palpatine´s ascent to Emporer, he purged knowledge of Thule from histories and the few sources that documented its existence. Thuke´s role as a hidden planet was to continue as a resource for the Sith, not the Empire.
Thule continued as a hiding place during The New Jedi Order era. Its dark side cover had hidden the planet from more skilled Jedi during the Old Republic, and the new batch had no hope of piercing its shroud. Not even the Yuuzhan Vong seem to know about uts existence, though whether they care, given the planet´s noninterference, is another matter.