Sapping, or undermining, was a siege method used in the Middle Ages against fortified castles.

Use in the Middle Ages

A mine was a tunnel dug under the walls of a castle. Once under the walls, sappers would build wooden structures to hold up the tunnel that they had made. The tunnel would then be filled with flammable material and set on fire. Later, explosives were used for greater effect. If the sapping was done well the wall above it would fall down creating an entrance for the attacking army.

Peter of les Vaux-de-Cernay recounts how at the battle of Carcassonne, during the Albigensian Crusade, "after the top of the wall had been somewhat weakened by bombardment from petraries, our engineers succeeded with great difficulty in bringing a four-wheeled wagon, covered in oxhides, close to the wall, from which they set to work to sap the wall" (les Vaux-de-Cernay, 53).

As in the siege of Carcassonne, defenders worked to prevent sapping by dumping anything they had down on attackers who tried to dig under the wall. Successful sapping usually ended the battle since either the defenders would no longer be able to defend and surrender, or the attackers would simply charge in and engage the defenders in close combat.

Use in the American Civil War

During the Siege of Petersburg, Union troops dug a tunnel under the Confederate lines and packed its end with vast amounts of gunpowder. When set off, the resulting explosion killed about 300 soldiers. It might have been decisive if not for faulty Union tactic of storming into, rather than around, the resulting crater, allowing the defenders to shoot down onto attackers unable to climb the steep crater sides. The combat was accordingly known as the Battle of the Crater.

Use in the Modern Era

Sapping saw a brief resurgence as a military tactic during the First World War when army engineers would attempt to break the stalemate of trench warfare by tunneling under no man's land and laying large quantities of explosives beneath the enemy's trench. As in siege warfare, mining was possible due to the static nature of the fighting.

A notable example was Messines Ridge, under which 450 tonnes of high explosive were placed in 21 mines after about two years of sapping. Approximately 10,000 German troops were killed when 19 of the mines were simultaneously detonated. One of the explosive caches exploded years later. The 21st cache was never found and there are still several tonnes of high explosive buried somewhere in the French countryside.

By World War II troop movements were too fluid, and tunneling too slow, for sapping to be worth the investment of effort. Sappers did see some use during the Vietnam War, such as when U.S. forces were holding Khe Sanh.