Rani Karnawati and Emperor Humayun
The story of Rani Karnavati and Emperor Humayun is the most significant evidence in the history. During the medieval era, Rajputs were fighting Muslim invasions. Rakhi at that time meant a spiritual binding and protection of sisters was foremost. When Rani Karnawati the widowed queen of the king of Chittor realised that she could in no way defend the invasion of the Sultan of Gujarat, Bahadur Shah, she sent a rakhi to Emperor Humayun. The Emperor touched by the gesture started off with his troops without wasting any time.
Alexander The Great and King Puru
The oldest reference to the festival of rakhi goes back to 300 B.C. at the time when Alexander invaded India. It is said that the great conqueror, King Alexander of Macedonia was shaken by the fury of the Indian king Puru in his first attempt. Upset by this, Alexander's wife, who had heard of the Rakhi festival, approached King Puru. King Puru accepted her as his sister and when the opportunity came during the war, he refrained from Alexander.
Lord Krishna and Draupathi
In order to protect the good people, Lord Krishna killed the evil King Shishupal. Krishna was hurt during the war and left with bleeding finger. Seeing this, Draupathi had torn a strip of cloth from her sari and tied around his wrist to stop the bleeding. Lord Krishna, realizing her affections and concern about him, declared himself bounded by her sisterly love. He promised her to repay this debt whenever she need in future. Many years later, when the pandavas lost Draupathi in the game of dice and Kauravas were removing her saari, Krishna helped her divinely elongating the saari so that they could not remove it.
King Bali and Goddess Lakshmi
The demon king Mahabali was a great devotee of lord Vishnu. Because of his immense devotion, Vishnu has taken the task of protecting bali's Kingdom leaving his normal place in Vikundam. Goddess lakshmi - the wife of lord Vishnu - has became sad because of this as she wanted lord Vishnu along with her. So she went to Bali and discussed as a Brahmin woman and taken refuge in his palace. On Shravana purnima, she tied Rakhi on King Bali's wrist. Goddess Lakshmi revealed who she is and why she is there. The king was touched by Her and Lord Vishnu's good will and affection towards him and his family, Bali requested Lord Vishnu to accompany her to vaikuntam. Due to this festival is also called Baleva as Bali Raja's devotion to the Lord vishnu. It is said that since that day it has become a tradition to invite sisters on sravan pournima to tie sacred thread of Rakhi or Raksha bandan.
Source : www.raksha-bandhan.com