Fire Time (Doubleday science fiction) Hardcover – January 1, 1974 by Poul Anderson
 
The planet Ishtar has three suns: Bel, the "real" sun, the Life Giver. Ea, the Companion who warms the Ishtaran summers. Anu, the Demon Star. Mostly Anu is so far away that it is just a light in the Ishtaran sky. But once every thousand years it comes close. It is then that the barbarians must flee their scorched lands, and civilizations fall. The natives call this Fire Time. Always before, its coming had meant the death of a civilization. But this time, the humans are here, and they have brought with them their magical technology. This time things could have been different. Too bad that the humans are suddenly faced with a war of their own, their own Fire Time.
 
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