The Thraddash are a race of intellectually challenged barbarians from the Draconis stars. They have built and decimated their civilization nineteen times by the time of Star Control II/The Ur-Quan Masters, naming their new civilizations numerically, i.e. Culture 19. They are best dealt with by tricking them into attacking the Kohr-ah, waiting for them to retreat to their space in diminished numbers, then pummeling them mercilessly into submission, where they will rename their civilization Culture 20 or a title of your choosing. Their alliance will make the acquisition of the Aqua Helix, an important artifact needed to repair the Ultron and ultimately beat the game, possible.
The Aqua Helix can also be taken while the Thraddash are away, attacking the Kohr-Ah. And even a third method exists: you can trick the Ilwrath into attacking them by broadcasting using a HyperWave Caster near their homeworld. You can then either take the Helix while the battle rages and the Thraddash are distracted, or wait until the races have fought each other long enough and then proceed to attack them yourself, as with the attack on the Kohr-Ah.