Nephite hubris blinds them to the reason for their success. They had defeated Lamanite armies thanks to a well-conceived defensive position. Nevertheless, they gave the credit to their military prowess instead, planned to leave that defensive position, and anticipated going “up to battle.”
Reference: The Nephites take an oath to attack. There are two aspects to this oath. First is the fact that they made the oath. The second is the specific wording of the oath: “they did swear by the heavens, and also by the throne of God.” This particular oath is found in the Sermon on the Mount: “But I say unto you, Swear not at all; neither by heaven; for it is God’s throne” (Matt. 5:34, 3 Ne. 12:34). Because the language is possibly culturally related to the Jews of the Old World, the actual wording of the oath may have had different cultural referents in the New World, represented by the more familiar oath. In verse 14 Mormon will emphasize that the Messiah has prohibited these oaths.