At some point in their long history, at a time when they’d reached a staggeringly high level of knowledge and technology, their planet was rife with conflict between many opposing polities. These conflicts threatened the survival of all life on the planet.
Political solutions were deemed impossible since no group trusted off of the other groups. And, no one would disarm their doomsday weapons for fear of being vulnerable to attack by coalitions of the other polities.
A great scientist built a machine that generated a planetary scale field which inhibited nuclear decay by modulating the weak and strong atomic forces in ingenious ways. The machine, once started, would become the ultimate doomsday weapon, and destroy the planet if turned off, but made all of the other doomsday weapons and nuclear devices useless.
Faced with a loss of their deterrents, and effectively protected against obliteration, the polities went mad fighting wars using conventional methods — bioweapons, chemical weapons, nano-tech, masers, lasers, blasters. After a thousand years of warfare, they found their own paths to peace, and formed a one world government.
Now, they want to leave their planet but the great machine still operates, preventing nuclear decay, and making nuclear engines inoperative within a few thousand miles of the surface of their planet