Jamieson, Fausset and Brown on 2 Samuel 10:4

4. Hanun took David's servants, and shaved off the one half of their beards--From the long flowing dress of the Hebrews and other Orientals, the curtailment of their garments must have given them an aspect of gross indelicacy and ludicrousness. Besides, a knowledge of the extraordinary respect and value which has always been attached, and the gross insult that is implied in any indignity offered, to the beard in the East, will account for the shame which the deputies felt, and the determined spirit of revenge which burst out in all Israel on learning the outrage. Two instances are related in the modern history of Persia, of similar insults by kings of haughty and imperious temper, involving the nation in war; and we need not, therefore, be surprised that David vowed revenge for this wanton and public outrage.