John Gill Commentary 2 Samuel 1:19

"Thy glory, O Israel, is slain upon thy high places! How are the mighty fallen!" — 2 Samuel 1:19 (ASV)

The beauty of Israel is slain upon your high places
The high mountains of Gilboa, where Saul their king, and Jonathan his son, a prince of the blood, and natural heir to the crown, and multitudes of young men, the flower of the nation, were wounded and slain. Here begins the lamentation, or the elegiac song:

how are the mighty fallen !
mighty men of war, strong and valiant, as Saul and his sons were, and the soldiers in his army.