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T>I'm asking about division of labor - which was the first society to have a group of people dedicated to defence, and who weren't expected to work in agriculture or trades?

he Imperial Roman military institution, it was probably the first ever ENTIRELY professional army. Although professional soldiers did exist in China, Persia, Egypt etc.; they usually made up a very small percentage of a campaign army. In a situation of emergency, or to embark on a military campaign; an army would be mustered by the levy of peasants (as in most Greek city states and the Roman republic); by hiring mercenaries (Carthage, Persia); or by calling up members of a determined "warrior caste" (who in peacetime engaged in non-military activities); but once the campaign was over, the bulk of the army would be disbanded. Many states such as Carthage and Ptolemy's Egypt, had been obliged to grant land to discharged mercenaries after major military campaigns, because they were unable to pay them a salary during peacetime.  The Imperial Roman army was a military institution in which ALL soldiers were career professionals and were paid a salary both in peacetime and during war; just for the sake of being soldiers.