Some from Manasseh defected to David when he went with the Philistines to fight against Saul. (They did not help the Philistines because the Philistine rulers consulted and sent David away, saying, "It will cost us our heads if he defects to his master Saul.")

1 Samuel 29:2
As the Philistine leaders marched out their units of hundreds and thousands, David and his men marched behind them with Achish.

1 Samuel 29:3
Then the commanders of the Philistines asked, "What about these Hebrews?" Achish replied, "Is this not David, the servant of King Saul of Israel? He has been with me all these days, even years, and from the day he defected until today I have found no fault in him."

1 Samuel 29:10
Now then, get up early in the morning, along with your master's servants who came with you, and go as soon as it is light."

1 Chronicles 12:20
When David went to Ziklag, these men of Manasseh defected to him: Adnah, Jozabad, Jediael, Michael, Jozabad, Elihu, and Zillethai, chiefs of thousands in Manasseh.

And there fell some of Manasseh to David, when he came with the Philistines against Saul to battle: but they helped them not: for the lords of the Philistines on advisement sent him away, saying, He will fall to his master Saul to the jeopardy of our heads.