Alma 9:16 - 9:20
Alma 9:16 For there are many promises which are extended to the Lamanites; for it is because of the traditions of their fathers that caused them to remain in their state of ignorance; therefore the Lord will be merciful unto them and prolong their existence in the land.
The Lamanites were be getting more breaks than the Nephites in the forgiveness department. They are sinning because they don’t really know any better being raised by parents who did not any better either. As a result, the Lord will allow them to live in the land of promise even in this condition.
Alma 9:17 And at some period of time they will be brought to believe in his word, and to know of the incorrectness of the traditions of their fathers; and many of them will be saved, for the Lord will be merciful unto all who call on his name.
In time, the Lamanites will be brought to a knowledge of who they really are and why they were in the condition they found themselves culturally and spiritually. Many will repent and be number with the saved in the kingdom of God.
Alma 9:18 But behold, I say unto you that if ye persist in your wickedness that your days shall not be prolonged in the land, for the Lamanites shall be sent upon you; and if ye repent not they shall come in a time when you know not, and ye shall be visited with utter destruction; and it shall be according to the fierce anger of the Lord.
However, things are different for the people of Ammonihah. They know better and if they refuse to repent of their wickedness, the Lord will send the Lamanite armies and destroy them and their city. They will strike when the Ammonihahites least expect it and it will be when the Lord so directs.
It might seem strange that the Lord would send armies to destroy His children, but it appears to me to be better that these people be in the spirit world then mortals. In addition, there is a point when the Lord says a person has had enough witnesses and they have shown their true colors and the test of mortality needs continue no more.
Alma 9:19 For he will not suffer you that ye shall live in your iniquities, to destroy his people. I say unto you, Nay; he would rather suffer that the Lamanites might destroy all his people who are called the people of Nephi, if it were possible that they could fall into sins and transgressions, after having had so much light and so much knowledge given unto them of the Lord their God;
The Lord simply cannot allow the Ammonihahites to live any longer among the Nephites. If He allowed this, the entire Nephite civilization would, in time, fall to their efforts to destroy the government. The Lord just will not allow that because the Nephites have had so much of the gospel in their lives. The Lord just will not stand for that to be destroyed by the Ammonihahites or anyone else.
Alma 9:20 Yea, after having been such a highly favored people of the Lord; yea, after having been favored above every other nation, kindred, tongue, or people; after having had all things made known unto them, according to their desires, and their faith, and prayers, of that which has been, and which is, and which is to come;
This is because the Nephites have been more favored by the Lord then any other civilization. This includes very civilization now in existence. They have had knowledge given to them that other nations have not be given. It is because of their great faith in the past, that they have had great blessing given to them to save them in battle because of their prayers of faith. They have had knowledge of things to come and of the gospel of Jesus Christ as well. The Nephites have truly been bless above all other peoples.