Of Manasseh also there fell away some to David, when he came with the Philistines against Saul to battle: but they didn't help them; for the lords of the Philistines sent him away after consultation, saying, He will fall away to his master Saul to the jeopardy of our heads.
"When he (David) came with the Philistines against Saul"  (1 Chronicles 12:19).  A full account of what is here mentioned is given in  1 Samuel 29. (See that reference for our notes regarding it.)  It was upon that occasion that some of the tribe of Manasseh united with David.  It was a timely addition to David's forces, because he fought a battle against the Amalekites immediately afterward.