Helaman 4:25

therefore the Lord did cease to preserve them by his miraculous and matchless power, for they had fallen into a state of unbelief and awful wickedness; and they saw that the Lamanites were more exceeding numerous than they, and except they should cleave unto the Lord their God, they must unavoidably perish.

So the Lord stopped protecting them with his amazing power. They had stopped believing and become very evil. They saw that the Lamanites had many more people than they did. Unless they stayed close to the Lord their God, they would die.

So the Lord stopped protecting them with his miraculous power. They’d fallen into unbelief and terrible wickedness. They saw that the Lamanites far outnumbered them, and unless they held fast to the Lord their God, they would surely be destroyed.