Mormon 2:1

And it came to pass that in that same year, there began to be a war again between the Nephites and the Lamanites. And notwithstanding I being young, was large in stature, therefore the people of Nephi appointed me that I should be their leader, or the leader of their armies.

That same year, a war started again between the Nephites and the Lamanites. I was young, but I was large and strong. So the Nephite people chose me to lead their armies.

That same year, war broke out again between the Nephites and Lamanites. Even though I was young, I was big for my age, so the Nephites made me their military commander.