Alma 43:36

And it came to pass that the Lamanites, when they saw the Nephites coming upon them in their rear, turned them about, and began to contend with the army of Lehi;

When the Lamanites saw the Nephites coming behind them, they turned around and started to fight against Lehi’s army.

When the Lamanites saw Nephites attacking from their rear, they turned and began fighting Lehi’s army.