Passive anti-tank defences: dragon's teeth and obstacles

Passive anti-tank defences are obstacles that do not shoot. They do not destroy vehicles; they slow them, divert them and force them to bunch up exactly where the defender has placed his weapons. Concrete dragon's teeth, steel hedgehogs, ditches and minefields form a single system whose purpose is not to stop an attack on its own, but to take away its speed and its freedom of choice.

That principle explains why a seemingly crude row of concrete blocks still matters on contemporary battlefields, and why its effectiveness depends entirely on what the defender places behind it.

What an obstacle that cannot shoot is for

Military engineering doctrine classifies obstacles by the effect they seek on the attacker: block, turn, fix or disrupt. An isolated obstacle achieves little of that for long, because it can be bypassed or breached with engineer equipment. Tied into a fire plan, the equation changes: an attacker looking for a passable route is led to a point known in advance, where the defender concentrates artillery, anti-tank missiles or, today, drones.

The second effect is time. Breaching means halting, deploying engineers and working under observation. That interval is precisely what the defence needs to bring up reserves. In practice, a belt of obstacles is worth whatever surveillance covers it: without observation and fire, it is merely a nuisance that costs a few hours.

The main families of obstacle

From the West Wall to the Atlantic Wall

Dragon's teeth are associated above all with the German Siegfried Line, the Westwall built in the 1930s along the Reich's western border, where belts of concrete pyramids were combined with bunkers and artillery positions. The same logic reappears in the Atlantic Wall, whose beach obstacles were meant to hold landing craft inside the beaten zone, and in the Swiss border fortifications still preserved as heritage today.

The Second World War left a lesson that historians of fortification repeat: none of those lines proved impassable. The real value of a fixed barrier lies not in its solidity but in whether the defender has mobile forces able to react where the barrier succeeds in slowing the attacker. Where those reserves were missing, the line fell in days.

Their return in current warfare

The war in Ukraine has put passive obstacles back at the centre of attention. The Russian defensive lines built across the south and east of the country, extensively documented through satellite imagery and by analysts at RUSI and the Institute for the Study of War, combine ditches, rows of dragon's teeth, wire, dense minefields and infantry positions in depth. Their main effect has been to cut the rate of advance at very high cost to the attacker.

One factor is new. Surveillance and strike drones have multiplied the value of obstacles, because a vehicle halted or bottled up in front of a barrier is an easy target located in real time. Breaching, always dangerous, has become deadlier: engineers opening a lane now work under permanent observation.

• An obstacle on its own does not defend: it works only when tied to observation and fire.
• Its output is measured in time gained, not vehicles destroyed.
• It is cheap to build compared with the equipment needed to breach it.
• It is visible and permanent, which lets the attacker plan in advance.
• After the war, the concrete remains — and above all the associated mines — keep the ground contaminated.