The request is made. Submit or be destroyed. This letter announcing the intent of the attacker does have parallel in Aztec practice, though it did not happen all of the time. It might have been more frequent in other cultures, but evidence is simply unavailable for those cultures or earlier time periods. In the Aztec practice, it was not a letter, but a personal ambassador, a difference doubtless due in part to the inability of the Aztecs to write a letter:
“The Aztecs often sent an ambassador to a foreign group to ask that it submit to Tenochtitlan and become a tributary of the Aztec state. If it refused, the area was a candidate for conquest, and the killing of an ambassador definitively meant war.” (Ross Hassig. Aztec Warfare. University of Oklahoma Press, Norman, 1988, pp.50-51).
While the mode of delivering the message is different, the intent is precisely the same. There is a formal invitation to submit prior to the battle.