This poem, a standard device, warns humans about thinking that social status, rank, royalty, etc. exempts one from the final fate of all.  The sentiment, not really addressed to those who have rank and bloodlines of nobility, is a consolation to those who have none—the thought that Death is a leveller of classes is both a soothing thought ans a warning that goodness, not rank, is what gets the soul to heaven.  The only armour against fate is “the actions of the just” and bloody battles, which seem so important now, are nothing but a part of the great cycle “and in the dust be equal made.”  The Renaissance poets constantly made this point, the difference between the “shadows” of our earthly state and the eternal state of our souls after death—that our earthly station was not important, but that our good deeds are (see Everyman, for example).  By comparing war to farming (“Some men with swords may reap the field...With the poor crooked scythe and spade") Sidney is equating the lowly man (the farmer) with no rank to the noble warrior, in that all are subject to “the great Leveller”; the garlands that grace the victor (a practice from the Greeks) also will wither, just as his fame withers.  The poem is fairly transparent, and its ABABCCDD rhyme scheme is standard ballad form.