It was a historic day for Earthlings and Resividans alike. First contact had been made on the latter’s planet and both sides wanted peace. Per ancient tradition, the leader of the Resividan delegation poured sacred water into the Peace Cup, and everyone drank from it. After the ceremony was complete, the Earthlings headed for home.
Unfortunately, germs each side naturally carried that were harmless on their home planet were deadly to those from the other planet. Within two weeks, everyone who drank from the Peace Cup was dead.
Both sides suspected the other of treachery and assassination via poison. Deciding diplomacy had failed, they then relied on something that had been an ancient tradition on both their planets.