Poul Anderson 
Let the Spacemen Beware 

To the men of the starship Quetzal their task seemed routine: to establish diplomatic relations and enter into trade negotiations with the people of a newly discovered planet. Since to all appearances the folk of Gwydion were the most thoroughly amiable in all the galaxy — words for such concepts as war, theft, anger, murder and jealousy did not even exist in their language — there could hardly be a problem. But as the season changed on Gwydion, so did the people…