We have discovered a habitable planet, possibly with liquid water oceans, 20 light years away. Shortly after the discovery, we beamed a high-intensity "good-will message" to the planet. Approximately 40 years from now, we can expect to be invaded by space aliens from Gliese 581d. Start preparing now, people.
We should have ended that good will message with "By the way, we have nukes so don't even try it."