The promise contained in verses 20 and 21 came to Nephi because of his faith, diligence and humility (verse 19). It represents a motto, a promise, and a charge to the Nephites. It becomes the great law by which Nephite nations are prospered or destroyed. This law is so important that it is repeated at least 10 times in the Book of Mormon. (1 Nephi 2:20, 2 Nephi 1:20, 2 Nephi 4:4, Jarom 1:9, Mosiah 2:31, Alma 9:13, 36:1, 36:30, 37:13, 38:1) This promise is still in force in our day.
Spencer W. Kimball
"This America is no ordinary country. It is a choice land, 'choice above all other lands' (1 Ne. 2:20). It has a tragic and bloody past, but a glorious and peaceful future if its inhabitants really learn to serve their God. It was consecrated as a land of promise to the people of the Americas, to whom God gave these great promises:
'It will be a land of liberty to its people' (2 Nephi 1:7).
'They shall never be brought down into captivity' (2 Nephi 1:7).
'And there shall be none to molest them' (2 Nephi 1:9).
'It is a land of promise' (1 Nephi 2:20).
'It shall be free from all nations under heaven.'
'There shall be no enemies come into this land.'
'It shall be free from bondage' (Ether 2:12).
'There shall be no kings upon the land' (2 Nephi 10:11).
'I will fortify this land against all other nations' (2 Nephi 10:11).
'He that fighteth against Zion shall perish' (2 Nephi 10:13).
"But these promises, glorious though they be, desirable as they are, can come only '…if they will but serve the God of this land who is Jesus Christ…' (Ether 2:12). There is only one way. That infallible cure is simply righteousness, obedience, Godliness, honor, and integrity. There is no other cure. Mountains of arms and ammunitions will not guarantee safety, for enemies can also build fortifications and missiles and bomb shelters. If we would but believe the prophets! For they have warned that if the 'inhabitants of this land are ever brought down into captivity and enslaved, it shall be because of iniquity; for if iniquity shall abound cursed shall be the land' (See 2 Nephi 1:7)." (Conference Report, Oct. 1961, pp. 30-31 as taken from Latter-day Commentary on the Book of Mormon compiled by K. Douglas Bassett)