About 20 years ago, I had a dream which I thought would make a good short story. I really like it, but I am only a very casual sci-fi fan, so I assume somebody has used this plot device before. I told it to someone once, and they said it reminded them a bit of some story but couldn't come up with it. Has this story been written?
The story is of a planet with many races, all constantly fighting against each other. For millennia, they were in a constant state of war. Over time, many races were vanquished, until there were only two races left. And one was annihilating the other.
Finally, the last people of the losing race were slaughtered. The triumphant race declared a month-long celebration. After the celebration was over... people looked around and didn't quite know what to do next. There were even reports of fighting between people within the same race, which previously was so taboo as to be unthinkable.
As things were starting to get more and more worrying, a group of astronomers announced a discovery: an alien race, of roughly similar technological advancement, was discovered on a distant planet. The military and the political leadership announced a plan to build an enormous colony ship to make the 500 year journey to the distant planet, to conquer them.
500 years later: the colony ship approaches the distant planet. Fighters are sent out to battle their forces. Surprisingly, the alien race doesn't fight back at all. As they slaughter the aliens, the invaders attempt to establish a communications link. At first they can only get audio, and they hear voices in an unintelligible but strangely familiar-sounding language, which seem to be begging for mercy. Finally they get the video connection working, and they are shocked to find that the alien race looks just like them!
They immediately send out an order to stop fighting. A middle-aged crew member nervously approaches the captain to tell him what his great-grandfather told him on his death bed, which he in turn heard from his own great-grandfather. There was a rumor, or urban legend, that the astronomers' announcement was a lie. The astronomers and politicians created this lie in order to distract the hostile populous.
Turns out, it worked better than they could have hoped. Once the colony ship set sail, the people's thirst for conquest was safely satisfied. Over the next centuries, their warlike tendencies subsided, eventually becoming a purely peaceful people. All of the research that formerly went into weapons was instead funnelled into peaceful scientific exploration. Eventually, they invented a much faster mode of interstellar travel, and they set out to explore the galaxy. They travelled to this planet and colonized it, well before the long-forgotten colony ship arrived.