Mahabharata - Why Arjun had to go to exile for 12 years

It is mentioned in the legend of Mahabharata that once Arjuna had to go to exile for 12 years due to breaking the rules.

This is about the time when Draupadi was married to the Pandavas and after marriage one day Devarshi Narada came to meet her and told her a story. In ancient times, Sunda and Usund were two demon brothers, they also defeated the gods with their valor, they did the harsh penance of Bramha ji, due to which they asked for a boon, both of them should be killed by each other. Both the brothers got boon and created chaos in the three worlds. All the gods were upset and went to Brahma ji when he asked Vishwakarma to create such a beautiful woman which everyone should be fascinated by seeing. Both the demon brothers were fascinated to see him and both fought fiercely for him and both brothers were killed.

Then the Pandavas noticed this and made a rule that from time to time every brother would stay with Draupadi for a certain time. When one brother is in solitude with Draupadi, no other brother will go and if anyone violates this rule he has to undergo 12 years of exile.

It is a matter of a day that the robbers robbed the cow of a Brahmin and that Brahmin came to Arjun and told him his pain. But the problem in front of Arjuna was that Yudhishthira was sitting with Draupadi in the room where his weapons were kept. And as per the rules, he could not enter that room. Then Brahma said that if you cannot protect your subjects then you are a sinner, then Arjuna pledged that he would have to protect that Brahmin even if he had to go to exile. After that Arjuna went to Yudhishthira's chamber and took his bow Raised and killed those robbers and handed the cow to the Brahmin. After this, Arjun came from Yudhishthira and said that I have broken my vow, so you can give me the command that I can go on exile for 12 years. During this exile, Arjuna married 3 girls, the first was Nag Kanya Ulupi, the second was Chitrangada, the princess of Manipur, and the third was Subhadra, the sister of Lord Krishna himself.