1 Samuel 18:20-21 (KJV):
And Michal Saul's daughter loved David: and they told Saul, and the thing pleased him. And Saul said, I will give him her, that she may be a snare to him, and that the hand of the Philistines may be against him. Wherefore Saul said to David, Thou shalt this day be my son in law in the one of the twain.
The idea that Michal being given to David will cause the Philistines to be against him seems to be consistent across Masoretic translations ["that she may be a snare to him, and that the hand of the Philistines may be against him" (KJV) -> "She'll be a trap for him, and the hand of the Philistines will be against him" (CSB) -> "that she may be a snare to him and so that the hand of the Philistines may be against him" (NIV).], but I'm uncertain why that would be. Why would David marrying Michal carry any consequences in relation to the Philistines?
I thought the Septuagint might help, but the MT's 'Michal + David = Philistines vs David' isn't present. Instead the Septuagint has (seemingly) separate statements -- Michal will be a stumbling block for David. The Philistines are against Saul. (v.20-21, Brenton Translation):
And Melchol the daughter of Saul loved David; and it was told Saul, and the thing was pleasing in his eyes. And Saul said, I will give her to him, and she shall be a stumbling-block to him. Now the hand of the Philistines was against Saul.