The Bodhisatta was once an ascetic. He lived in the Himalayas and near his hut a mongoose lived in an ant-hill and a snake in a hollow tree. The two animals were always quarreling, so the Bodhisatta successfully preached the value of harmony and the pettiness of quarreling to them and they began to get along well. One day when the serpent had gone out, the Bodhisatta saw the mongoose sleeping in the entrance of his burrow with his mouth open and asked why he was afraid of the snake. The mongoose answered that he no longer despised the snake, but he still mistrusted him. The Bodhisatta assured him he had nothing to fear anymore, so the mongoose stopped being suspicious.
In the Lifetime of the Buddha
The snake and the mongoose were earlier births of two high-ranking royal military officers who hated each other and spoke rudely every time they met. Neither the king nor their friends and family could set them right. One day the Buddha divined that these two men were close to having a spiritual breakthrough, so the next morning he went out collecting alms at each of their houses. While sitting with them, the Buddha preached about loving-kindness and dharma so clearly they both became disciples. The two soldiers forgave each other and were harmonious from then on.
Later, when the Buddha heard some of his disciples discussing how he had humbled the two soldiers, he told them this story so they knew he had also reconciled the same two men in previous lives.