Adam Clarke on Genesis 26:15

For all the wells - the Philistines had stopped them - In such countries a good well was a great acquisition; and hence in predatory wars it was usual for either party to fill the wells with earth or sand, in order to distress the enemy. The filling up the wells in this case was a most unprincipled transaction, as they had pledged themselves to Abraham, by a solemn oath, not to injure each other in this or any other respect. See Gen 21:25-31.