What's The Most Disastrous First Encounter Between Two Civilizations?

First contact can be a delicate business. When humans encounter aliens (or fairies, or other creatures) for the first time, it can easily go very, very wrong. Any first meeting between two peoples can turn bloody at a moment's notice. But what's the most horrific first contact scenario in science fiction and fantasy?

Please include a picture, clip or book cover for your chosen diplomatic disaster. And please mention the name of the peoples who met with terrible consequences, what story this was in, and why it was so bad. Thanks!

Top image: Babylon 5, the Earth-Minbari war, art by TimeWyrm.