Lehi was a Nephite military commander under Captain Moroni during the wars of the book of Alma. He first appears leading the army concealed south of the hill Riplah; as the Lamanites crossed the river Sidon, he led his men forth and encircled them in their rear, and they fled toward the river (Alma 43:35, 39).
Moroni later appointed Lehi chief captain over the city of Noah. When the Lamanites found that Lehi commanded the city they feared him, but had sworn to attack; they were driven back from the fortifications with more than a thousand of them slain, while not a single Nephite was killed and about fifty were wounded (Alma 49:16-24).
At the city Mulek, the Lamanites pursued Teancum until they were met by Lehi and a small force defending Bountiful. Lehi did not overtake them until they had run into Moroni’s army, and the Lamanites were caught between the men of Moroni and the men of Lehi. Lehi pressed their rear until they gave up their weapons (Alma 52:27-36). Moroni then went with Lehi to Mulek, took command of the city, and gave it to him (Alma 53:2).
Near the end of the war, Moroni, Lehi, and Teancum encamped around the land of Moroni and encircled the gathered Lamanite armies (Alma 62:33-34). When Moroni left to deal with the rebellion in the Nephite government, he gave Lehi and Teancum command over the remainder of his army (Alma 62:3). After the war, when Coriantumr led a Lamanite army through Zarahemla, Moronihah sent Lehi with an army to head them off before they reached the land Bountiful (Helaman 1:28-29).
The record describes Lehi as a man who had been with Moroni in most of his battles, “a man like unto Moroni,” and beloved by Moroni and by all the people of Nephi (Alma 53:2).