Charles Ellicott Commentary 1 Chronicles 12:19

"Of Manasseh also there fell away some to David, when he came with the Philistines against Saul to battle: but they helped them not; for the lords of the Philistines upon advisement sent him away, saying, He will fall away to his master Saul to the jeopardy of our heads." — 1 Chronicles 12:19 (ASV)

There fell. —The regular term for desertion of one cause for another (2 Kings 25:11).

When he came with the Philistines.—(Compare to 1 Samuel 29:2–11.) This verse is a summary of the narrative of 1 Samuel 29:2 to 1 Samuel 30:1.

They helped them not. —David and his men did not help the Philistines. Perhaps the right reading is he helped them (‘azarâm), not they helped them (‘azarûm).

Upon advisement. —After deliberation (Proverbs 20:18).

To the jeopardy of our heads. —At the price of our heads (1 Chronicles 11:19). By betraying us he will make his peace with his old master.