In "Rikki-Tiki-Tavi" by Rudyard Kipling, whom does the hero fight in the "great war"? Why does he fight?

Rikki-tikki fights the two cobras, Nag and Nagaina, because they threaten the people.

The story opens by telling us that we are going to hear about a war.

This is the story of the great war that Rikki-tikki-tavi fought single-handed, through the bath-rooms of the big bungalow in Segowlee cantonment. Darzee, the tailor-bird, helped him, and Chuchundra, the musk-rat ... gave him advice; but Rikki-tikki did the real fighting.

The war that Rikki-tikki fought was a war against snakes.  Rikki-tikki fought this war because, as a house mongoose, it was his job to protect the people and the garden from the snakes.  Rikki-tikki was brave and successful, because he defeated all of the snakes.

When Rikki-tikki washed up in the garden, the people were pleased and the cobras were not.  Rikki-tikki worried Teddy’s mother at first, because she thought the mongoose might be dangerous.  Teddy’s father assured her that the real danger was to the snakes.  Teddy was safer with Rikki-tikki there.

Rikki-tikki used cunning and perseverance to defeat the cobras.  He never let his guard down.  Even when Rikki-tikki killed a snake, he did not eat it because he did not want to be slowed down.  Thus, when he kills the little snake Karait, Rikki-tikki leaves the corpse.

That bite paralysed Karait, and Rikki-tikki was just going to eat him up from the tail, after the custom of his family at dinner, when he remembered that a full meal makes a slow mongoose, and if wanted all his strength and quickness ready, he must keep himself thin.

Rikki-tikki kills Nag first, and then Nagaina.  Nagaina is harder to kill because she has nothing left to lose at that point.  Her husband is dead, and Rikki lures her out by killing her baby cobras.  Rikki-tikki has to follow her into her hole, which is very dangerous. However, he emerges victorious.