Poul Anderson 
People of the Wind 

Like two giants the old enemies faced each other across the reaches of the galaxy — the Terran Empire and the Ythrian Domain. Terra was a Leviathan, encroaching ever further among the stars, promising peace and prosperity — but at the price of freedom. Ythri was smaller, but an empire in its own right, peopled by birdlike beings with a civilisation and intellect that easily matched Terra’s own.And between the adversaries lay Avalon. One single planet, inhabited by human and Ythri alike. Both sides wanted to claim Avalon, by persuasion or by force, for it was a key world that could turn the tide of the war. But Avalon had developed a unique culture, a powerful blend of human and Ythrian thought. And Avalon had ideas of its own….