Wickedness

Mormon 4:10,18

"And it came to pass that the three hundred and sixty and sixth year had passed away, and the Lamanites came again upon the Nephites to battle; and yet the Nephites repented not of the evil they had done, but persisted in their wickedness continually.

And from this time forth did the Nephites gain no power over the Lamanites, but began to be swept off by them even as a dew before the sun."

Because the Nephites continued in their wickedness, the Lord no longer protected them against the Lamanites. In fact, the Lord allowed the Nephites to be swept off because of their extreme wickedness. The wicked who delight in killing and other evils do not have the Spirit with them. They do not have that divine guidance, and so are more likely to be destroyed.