Flanks and Flanking
The end of an army's line is called the flank. A contiguous army’s line, therefore, has two flanks—one on the right and one on the left. The unit that is positioned on the very end of either side of the line sits on the extreme right or left flank. The end of the line is inherently a vulnerable military position, subject to turning movements and often devoid of support. To counter this reality, army flanks were often placed at strong defensive positions on high ground or behind watercourses.
Opposing forces regularly endeavored to exploit the vulnerable flanks. If an enemy moved a unit around your flank so that it could shoot at your men from the side or rear, it is called a flanking maneuver. You have been "flanked." When you are flanked you usually have two choices—leave or get shot. Inasmuch as Civil War soldiers did not have death wishes (as is often perceived) units that were flanked regularly fell back and took up a new position, bending back to face the new threat.