Rani Karnawati and Emperor Humayun
During the medieval era, Rajputs were fighting Muslim invasions. Rakhi at that time meant a spiritual binding and protection of sisters was foremost. A famous incident relates how rakhi by then had broken the religious barriers.

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.