It was only after the king finished this list of assurances, well prepared ahead of time, that the chief priest struck him hard upon the cheek, with an open hand but as strongly as he could. The blow had to be decisive and hard, for according to tradition tears had to flow from the king’s eyes as an indication that Bel (and his wife Beliya) were friendly, an omen which purported to assure king’s future success and the prosperity of the country. If there were no tears, this signified that Bel was angry, and thus that enemies were expected to rise up and bring about the king’s downfall. It is not known whether the rite could be repeated if tears failed to appear at the first stroke. But if the performance was satisfactory, and there was a steady flow of tears, then the arms, the scepter and the crown were restored to the king, who was now expected to be prosperous and could rule safely for another year.As for the political reflections, we've all encountered bureaucrats and politicians whom we thought could benefit from slapping, but it doesn't seem to have made the ancient Mesopotamian rulers notably more humble.
The priests of the huge Assyrian or Babylonian temples were rather a sophisticated lot. Their knowledge of writing, astronomy and the basic rules of a prosperous religious establishment, their role as teachers, top officials in a good and efficient government, secured for them the top position on the social scale. They were the real power behind the throne and could postpone a new king’s official coronation and recognition for several years, until he had proved himself successful in battle, in the taxation of his own people and the well-organized armed plunder of foreign lands. The priests and their temples were the first to receive a share (the largest) of the tribute and plunder: innumerable slaves, gold, silver, wood, concubines and singers brought to them by the king’s conquests and robberies of the foreign lands. The priests taught the king and his chosen ancestors how to discipline the people and the army, all for their own benefit.
However, the humiliation of the king during the New Year ritual served a double purpose: It demonstrated to the king that without his crown, sword and scepter he was just another ordinary mortal, whose fate depended on the mighty gods and their humble servants.
He might have been all-powerful, ruling over the entire world, but the pain of being hit in the face in this manner was meant to make him humble, more aware of his duties and obligations, inspiring him to take care of his promises, or face consequences.
UPDATE: A couple of earlier posts on the Akitu Festival are here and here.
UPDATE (1 October): More here.