Uatu is well-known for violating his oath of non-interference, most famously in Fantastic Four #48, but I remember a comic where a group of alien invaders set up weapons right next to the Watcher saying that they knew he would not interfere with their plans of conquest and destruction on the planet. Presently, and with a simple wave, he wipes them out saying what they didn't know was that the planet was his home world. What was that comic?
His home planet is T-37X.
At the time, Tales of Suspense usually had 3 stories, with the third being "Tales of the Watcher".
Here is a synopsis:
The Watcher was on a moon orbiting planet T-37X when he was spotted by a band of roving space pirates out to loot a world's natural resources. After determining T-37X contained large quantities of gold and uranium, they settled down on the moon to build a delta-ray cannon that would force the world to surrender. They incorrectly figured the Watcher would not intervene. Instead, Uatu disabled the cannon and banished the marauders to a distant world. He explained to the stunned looters that while his oath prevented him from interfering with other races, he was, however, free to act in the defense of his own, revealing that T-37X is the ancient home planet of the Watchers.