This is the key takeaway of our new book - "The Seventh Avatar".
The Seventh Avatar is story of the birth of Rama. Vishnu had to incarnate as Rama to end Ravana's atrocities.
Ravana was very successful and powerful. He was very talented and learned as well. There was one problem with him, however. His success got to his head disabling him from thinking straight. When he prayed to Brahma for a boon, he requested protection from all life forms except man. He was too conceited to believe that a man could actually harm him. Hence Vishnu took an avatar as a man to vanquish Ravana.
In history, this mistake of underestimating your opponent has been made time and again...