Here historical know how of rakhi festival is explained. This can be taken as hints for writing very special essay on raksha bandhan.
King Bali and Goddess Laxmi:
According to a legend the Demon King Bali was a great devotee of Lord Vishnu. Lord Vishnu had taken up the task to guard his kingdom leaving his own dwelling-place in Vaikunth. Goddess Lakshmi wished to be with her lord back in her dwelling-place. She went to Bali disguised as a woman to seek refuge till her husband came back.
During the Shravan Purnima celebrations, Lakshmi tied the sacred thread to the King. Upon being asked, she revealed who she was and why she was there. The king was touched by her goodwill for his family and her purpose and requested the Lord to accompany her. He sacrificed all he had for the Lord and his devoted wife.
It is said that since then it has been a tradition to invite sisters in Shravan Purnima for the thread tying ceremony or the Raksha Bandhan.
Rani Karnavati and Emperor Humayun:
A popular narrative that is centered around Rakhi is that of Rani Karnavati of Chittor and Mughal Emperor Humayun. When Rani Karnavati, the widowed queen of the king of Chittor, realised that she could not defend against the invasion by the Sultan of Gujarat, Bahadur Shah, she sent a Rakhi to Emperor Humayun. Touched, the Emperor immediately set off with his troops to defend Chittor.Humayun arrived too late, and Bahadur Shah managed to sack the Rani's fortress. When Humayun reached Chittor, he evicted Bahadur Shah from fort and restored the kingdom to Karnavati's son, Vikramjit Singh.
Alexander the Great and King Puru:
According to one legendary narrative, when Alexander the Great invaded India in 326 BC, Roxana (alexander's wife) sent a sacred thread to Porus, asking him not to harm her husband in battle. In accordance with tradition, Porus, gave full respect to the rakhi. On the battlefield, when Porus was about to deliver a final blow to Alexander, he saw the rakhi on his own wrist and restrained himself from attacking Alexander personally.