The hubris of the Nephites blinds them to the reason for their success. They have defeated Lamanite armies with a well-conceived defensive position. Nevertheless, they begin to think that it is their military prowess rather than the position that has been effective. Therefore, they begin to think that rather than defend, they will “go up to battle.”
Reference: The Nephites swear oaths that they will fight. This is, of course, not unusual for ancient peoples to make an oath as they enter battle. What we have in this case is two facets of the verse. The first is the fact of the oath, and fact that is completely at home in the Mesoamerican world. The second is the description of the oath. While it is possible that these particular oaths were the very ones that they swore, it is more likely that their oaths would be more culturally bound. These are the oaths that come from the Sermon on the Mount:
Matthew 5:34
34 But I say unto you, Swear not at all; neither by heaven; for it is God’s throne:
Mormon will explicitly point out that these oaths were prohibited by the Savior in verse 14 below. See the commentary following that verse for more information on this verse.