Read this post about an introduction to the Rasas.
When the Pandavas were in exile, Yudhisthira was performing the Rajarsi sacrifice with the help of some learned Brahmins on the shore of a lake in Dwaitavana. Duryodhana who happened to be passing by suddenly noticed the Pandavas on the lake shore and egged on by Karna decided to put on a show of his opulence and wealth to make the Pandavas jealous. He instructed his men to construct pleasure houses on the opposite shore of the lake. However, his men were prevented from doing so by a Gandharva king, Chitrasena who claimed the lake and its surrounding areas to be his.
Enraged by this interruption of his plans, Duryodhana attacked the Gandharva army with his own and suffered a humiliating defeat despite fighting valiantly and valorously. Duryodhana, his brothers and Karna were then taken prisoner by Chitrasena. Some of the soldiers who had escaped the Gandharva army fled to the other side of the lake and narrated what had happened to Yudhisthira and his brothers.
Upon hearing this, Arjuna and Bhima started laughing at the plight of their Kaurava cousins. Both of them were happy that their duties had become that much easier with Duryodhana and his brothers out of the way. In their opinion, Duryodhana got what he deserved as he surely had camped on the opposite side of the lake with evil intentions only.
To this, Yudhisthira intervened and told them that this was not the time for cruelty. He stated that it was the duty of family members to protect and uphold the honor and dignity of the family name. Despite the fact that the Kaurava cousins had treated him and his brothers shabbily, the fact remained that it was a family dispute and should not be used as a reason not to help them when they were in need of assistance. After all, all of them were Kuru princes and they were duty bound to defend the honor of their family name.
Saying so, Yudhisthira instructed Arjuna to battle the Gandharvas and free the Kauravas. Since he himself was bound by the sacred vows that he had taken before starting the Rajarsi sacrifice, he could not go into battle himself, but he was compassionate enough towards the Kauravas to send his brother to do the job.
Arjuna accompanied by Bhima and the twins, then attacked the Gandharva army and after a fierce battle managed to defeat the Gandharvas and free the Kauravas.
Thus, Duryodhana who had come to Dwaitavana to humiliate the Pandavas with his show of opulence and wealth had to go back to Hastinapura after being humiliated himself.
This incident is a clear case of Yudhisthira displaying compassion and kindness towards his sworn enemies, the Kauravas and is an example of the karunya rasa referred to in the Mahabharata. Despite the fact that he was duty bound to defend his family name, Yudhisthira could have easily avoided the same on some pretext or the other, but the fact that he chose not to displays his commitment to his duty and his compassionate nature.