1 Chronicles 12:19

“And there fell some of Manasseh to David, etc.] Of the tribe of Manasseh; they took his part, and on his side, and joined him: when he came with the Philistines against Saul to battle; which was a strong proof of their cordial attachment to him: but they helped them not: the Philistines; neither David nor his men, nor the Manassites that joined them: for the lords of the Philistines, upon advisement; counsel taken among themselves: sent him away; that is, David and his men: saying, he will fall to his master Saul, to the jeopardy of our heads; meaning, that he would go off with his troops to Saul, and betray them into his hands, and with their heads make his peace with him, (see <092904>1…”

“there fell some of Manasseh--The period of their accession is fixed as the time when David came with the Philistines against Saul to battle. but they helped them not--(See on Sa1 29:4).”

“when he came: Sa1 29:2-4 to the jeopardy of our heads: Heb. on our heads 1 Chronicles 12:20”

“The Manassites who went over to David before the last battle of the Philistines against Saul. - על גפל, to fall to one, is used specially of deserters in war who desert their lord and go over to the enemy: cf. Kg2 25:11; Sa1 29:3. אל יפּול, in the last clause of the verse, is a synonymous expression. The Manassites went over "when David went with the Philistines against Israel to the war, and (yet) helped them not; for upon advisement (בּעצה, cf. Pro 20:18), the lords of the Philistines had sent him away, saying, 'For our heads, he will fall away to his master Saul.' " Sa1 29:2-11 contains the historical commentary on this event. When the lords of the Philistines collected their forces to…”