Agency is a tremendous tool to allow our souls to be re-formed in the image of God. It is, however, a two-edged sword against which we may also run our souls. When agency is coupled with humility, we are able to learn from our life experiences, and turn experience into divine understanding. This process absolutely requires repentance. Repentance is the mechanism that turns experience into expansion; that alters the sin from barrier to springboard.
The Nephites were in a position where their sin was precisely a barrier to not only their spiritual progress, but in a very real sense, to their temporal existence. Rather than humbly turning to their Lord, they refused to hear the Lord. Instead, they believed in defenses of earth and wood rather than defenses of the infinite power of God. Of this sad tendency of man, Nephi told us:
2 Nephi 28:31
31 Cursed is he that putteth his trust in man, or maketh flesh his arm, or shall hearken unto the precepts of men, save their precepts shall be given by the power of the Holy Ghost.