3 Nephi 2:11

And it came to pass in the thirteenth year, there began to be wars and contentions throughout all the land: for the Gadianton robbers had become so numerous, and did slay so many of the people, and did lay waste so many cities, and did spread so much death and carnage throughout the land, that it became expedient that all the people, both the Nephites and the Lamanites, should take up arms against them;

In the 13th year, wars and fights began throughout all the land. The Gadianton robbers had become so many. They killed so many people. They destroyed so many cities. They spread so much death throughout the land. So all the people had to fight against them. Both the Nephites and the Lamanites took up weapons against them.

In the thirteenth year, wars and fighting broke out across the land. The Gadianton robbers had grown so numerous and so violent—killing thousands, destroying cities, spreading death and carnage everywhere—that everyone, both Nephites and Lamanites, had to take up arms against them.