Flank

A flank is the side of a military formation or position, as opposed to its front, which faces the enemy, or its rear, which lies behind it. The flanks are where a force is most vulnerable, because units are typically oriented and armed to fight to their front, where they expect the enemy, so an attack that strikes the side catches the force poorly positioned to respond, its weapons pointing the wrong way, its defenses arranged against a threat from ahead rather than beside. For this reason, the flank has been one of the most important concepts in warfare for as long as armies have formed up to fight, and the flanking maneuver, moving around the enemy's side to attack where they are weak, is among the oldest and most decisive tactics in the history of war.

Why flanks decide battles

The power of a flank attack comes from hitting the enemy where they are unprepared and where the attack unhinges their whole position. A force struck in the flank faces a threat its formation was not built to meet, and beyond the immediate tactical disadvantage, a successful flank attack can roll up a line, collapse it from the side, and threaten the rear, cutting off retreat and supply, which is why flanking maneuvers have produced some of history's most lopsided victories, Hannibal's double envelopment at Cannae, where he enveloped both Roman flanks and destroyed a much larger army, being the classic example studied by soldiers ever since. The logic scales from the smallest infantry engagement, where a squad maneuvers to fire into the side of an enemy position rather than assaulting its front, to the grandest strategic movements, where entire armies swing wide to strike an enemy's flank or rear, and the German offensive through the Ardennes in 1940 that outflanked the main French defenses is a modern illustration of a flanking move at the strategic level winning a campaign. The reason flanking works so consistently is rooted in the fundamental asymmetry it exploits: forces are strongest to their front and weakest to their sides and rear, so an attacker who can get onto the flank fights the enemy at a profound disadvantage, and much of tactics and strategy is a contest over the flanks, each side trying to turn the enemy's flank while protecting its own.

Because flanks are so dangerous, protecting them is a constant preoccupation of military planning. Forces anchor their flanks on obstacles that prevent the enemy from getting around them, a river, a mountain, the sea, an impassable feature, so that there is no open flank to be turned, and where no natural anchor exists, they extend their line, position reserves to counter a flanking move, or refuse a flank, bending it back to face a possible threat from the side. The security of the flanks shapes how forces deploy and maneuver, and a commander who leaves a flank exposed, or who fails to notice the enemy moving to turn it, invites the kind of disaster that flanking attacks have inflicted throughout history. Conversely, creating or finding an open enemy flank, by maneuver, by stretching the enemy, or by penetrating and then turning, is a central aim of offensive tactics, because the flank is where a decisive blow can be struck.

Flanks in modern war

The concept of the flank endures in modern warfare, though its expression has evolved with the changing character of the battlefield. In an era of long-range fires, drones, and dispersed operations, the neat lines with clear flanks of older warfare are often replaced by more fluid, non-contiguous fronts where the notion of a flank is less about a physical line's edge and more about the vulnerable directions from which a force can be struck, but the underlying vulnerability, that forces are weakest to their sides and rear, remains, and maneuvering to strike the enemy from an unexpected and poorly defended direction is as decisive as ever. Ukraine has seen flanking and the threat of it shape operations, with breakthroughs and withdrawals often turning on whether a flank could be turned or had to be protected, and the deep strikes that characterize modern war can threaten a force's flanks and rear from a distance, extending the flanking concept beyond the ground maneuver that traditionally embodied it. The word also carries into naval and air contexts and into strategic geography, where a nation or alliance speaks of its flanks, NATO's eastern flank being a prominent current example, meaning the vulnerable sides of its overall defensive position that must be secured against a threat, so the flank remains a living concept at every level from the infantry fireteam to the grand strategy of alliances.

What does it mean to turn an enemy's flank?

To turn an enemy's flank means to maneuver around the side of their position or formation so as to attack them from the flank or rear, rendering their frontal defenses useless and forcing them either to fight at a severe disadvantage, to bend their line back to meet the new threat, which weakens and disrupts their position, or to withdraw to avoid being enveloped and cut off. Turning a flank is one of the classic aims of offensive maneuver, because it achieves a decisive advantage without the costly frontal assault against the enemy's strongest point, striking instead where they are weak and unhinging their whole defense, and a force whose flank has been turned is often compelled to retreat or risk destruction, since remaining in place means being attacked from a direction it cannot effectively defend. The art of turning a flank lies in moving forces around the enemy's side faster than the enemy can react and reorient, which requires mobility, surprise, and often deception to fix the enemy's attention to their front while the flanking force moves, and the history of warfare is in large part a history of commanders seeking to turn their enemies' flanks while preventing their own from being turned.