Alma 54:6 - 54:10
Alma 54: 6 Behold, I would tell you somewhat concerning the justice of God, and the sword of his almighty wrath, which doth hang over you except ye repent and withdraw your armies into your own lands, or the land of your possessions, which is the land of Nephi.
Moroni wants Ammoron to know that God is watching what he and the Lamanite army are up too. And unless he returns to their own country the land of Nephi, the wrath of God will come upon them.
Alma 54: 7 Yea, I would tell you these things if ye were capable of hearkening unto them; yea, I would tell you concerning that awful hell that awaits to receive such murderers as thou and thy brother have been, except ye repent and withdraw your murderous purposes, and return with your armies to your own lands.
He would want to remind them of the hell he, his brother (Amalickiah) and others of a similar nature are to experience in the future if he does not withdraw from Nephite territory.
Alma 54: 8 But as ye have once rejected these things, and have fought against the people of the Lord, even so I may expect you will do it again.
Even if they did withdraw they would probably be back, just cannot help themselves.
Alma 54: 9 And now behold, we are prepared to receive you; yea, and except you withdraw your purposes, behold, ye will pull down the wrath of that God whom you have rejected upon you, even to your utter destruction.
Moroni tells Ammoron that the Nephites are ready for them to come at them again. And if they do, God will pour out his wrath on them with the power of the well trained and equipped Nephite armies under his command.
Alma 54: 10 But, as the Lord liveth, our armies shall come upon you except ye withdraw, and ye shall soon be visited with death, for we will retain our cities and our lands; yea, and we will maintain our religion and the cause of our God.
In the not too distant future, if the Lamanites do not withdraw, Moroni will attack and destroy them, in order to retain their country, religion and their rights.