It is said that when Xerxes, King of Persia, reviewed his magnificent and enormous army before starting for Greece, he wept at the thought of slaughter about to take place. “Of all this multitude, who shall say how many will return?” Emerson, in his English Traits, chap. iv., speaks of the Emperor Charlemagne viewing the fleet of the Norsemen in the Mediterranean Sea with tears in his eyes, and adds, “There was reason for these Xerxes' tears.”
Xerxes wept at the prospective loss he expected to suffer in the invasion prepared, but Charlemagne wept at the prospective disruption of his kingdom by the hardy Norsemen.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894