I read a short story online a year or so ago. Told from an alien's point of view, they discover Earth and are so horrified by mankind's warlike nature they fire a weapon at Earth that will take a few centuries to hit us.

They then watch as we evolve beyond the middle ages and realize we're not that warlike. The scientists involved in creating the weapon kill themselves by 'Sealing their spiracles' I seem to remember.

It ends with humanity developing fast enough to stop the weapon, then sending a message to the aliens along the lines of "big mistake".

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    I think I saw the answer to a similar question recently.
    – SQB
    Feb 22 '17 at 16:02
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  • Not directly related, but the basic concept underlies Harry Turtledove's WorldWar and Colonization series; aliens scouted Earth during medieval times; and expected the same basic level of technology when they returned in the 1940's. In this case, the aliens that arrive still try to conquer Earth
    – RDFozz
    Nov 1 '17 at 21:44

This is Gift of Mercy by anonymous, as mentioned on this other answer. Notably, the aliens express sorrow by "seal[ing] their spiracles with paste".

The Gift arrived suddenly, the light of its impact visible in our skies, shining bright and cruel even to the unaugmented ocular receptor. We watched and we wept for our victims, dead so many Deelis before the light of their doom had even reached us. Many 6^4s of those who had been directly or even tangentially involved in the creation of the Gift sealed their spiracles with paste as a final penance for the small roles they had played in this atrocity. The light dimmed, the dust cleared, and our Observatories refocused upon the place where their shining blue world had once hung in the void, and found only dust and the pale gleam of an orphaned moon, wrapped in a thin, burning wisp of atmosphere that had once belonged to its parent.

  • That's it, thanks!
    – evilscary
    Feb 23 '17 at 9:55

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