On March 15, kick-off day of Operation Undertone, an attack on German positions along a 75 km line from Saarbrücken to Haguenau, 1945 Herrera's unit found itself engaged in combat in a forest in the vicinity of the Bas-Rhin town of Mertzwiller, 5 miles northwest of Haguenau. His platoon came under heavy enemy machine gun fire from the woods, forcing most of the men to seek cover. Herrera charged the enemy stronghold single-handedly, firing his M1 rifle from his hip and ending the threat with a pair of grenades. Eight enemy soldiers threw down their weapons and surrendered.