Jamieson, Fausset and Brown on 2 Samuel 1:21

21. let there be no dew, neither let there be rain--To be deprived of the genial atmospheric influences which, in those anciently cultivated hills, seem to have reared plenty of first-fruits in the corn harvests, was specified as the greatest calamity the lacerated feelings of the poet could imagine. The curse seems still to lie upon them; for the mountains of Gilboa are naked and sterile.

the shield of the mighty is vilely cast away--To cast away the shield was counted a national disgrace. Yet, on that fatal battle of Gilboa, many of the Jewish soldiers, who had displayed unflinching valor in former battles, forgetful of their own reputation and their country's honor, threw away their shields and fled from the field. This dishonorable and cowardly conduct is alluded to with exquisitely touching pathos.