Fire Time by Poul Anderson
The planet Ishtar has an atmosphere breathable by humans. They have made a settlement on this unusual planet and have interacted with the native intelligent beings. Ishtar has three suns, one of which is a rogue star that swings near enough to the planet to scorch it every thousand years. This star, Anu, is nearing Ishtar and the northern land masses are heating up.
The natives of the planet are going to war. Those “barbarians” from the north are getting burned out. They are on the march to reclaim southern land that was theirs before. Those members of “The Gathering” in the south want to retain lands they have claimed for a thousand years.
The human settlement is in the south. Humans have non-interference laws that will not allow them to take sides in the war or to give advanced technology to the natives. Yet they can help with current technology to improve the planet’s current condition. Unfortunately, Earth is at war with another planet. All support has been withdrawn from the natives and has been redirected into a military base that is to be built on Isthar.
The centauroid type natives are very interesting. Other than that, I did not get pulled too far into the book. My interest was caught, never held for any length of time. All the elements are there, they just never jive right for me.