Planet of Exile, by Ursula K. Le Guin
The story is set at the coming of winter, and the migratory natives are seen for the first time to be banding together into large armies, which threatens the security of both the native cities and the colonists. By working together they have a chance to save themselves, but are nearly lost when their inability to see each other as equals destroys their alliance; the breakup sparked by the discovery that the daughter of a native chief has fallen in love with one of the leaders of the colony, and he returns her love. In the end they manage to fight together and fend off the migrating hordes, and it is seen that in addition to being accepted by the natives the settlers may be beginning to be accepted by their new world; they are adapting to, and becoming adapted to, its ecology to the point where they have a chance to survive.
It could all be very schmaltzy, but it isn't. Instead of miraculously being brought together by the love of the couple, the two tribes are - alas, far more realistically - nearly destroyed by the jealosy and racism it brings out. And they know that their love is foolish and self-destructive, but there just isn't much they can do about it. The panic and terror of fighting for their lives comes across very well - this is no glorification of battle - and yet so does the exhiliration of survival and victory. All throughout the story the question continually arises: "do we risk fighting the enemy only to lose to nature's winter?" and in the end it becomes clear that nature will indeed be the deciding factor. But luck gives them a brief symbolic victory against nature, which leads nicely into the chance that they might be able to continue to exist on this planet after all.
Ursula once again in fine form.