Tunnels have been used in warfare for thousands of years. Jewish rebels used tunnels against Roman legions, while the Viet Cong used them against US troops in southeast Asia. In Gaza, tunnels have enabled the flow of essential supplies, such as food and medicine, into the strip, as well as weapons.

Trenches used by German, French, British and Australian troops during World War I provided an efficient way for soldiers to protect themselves against heavy firepower, and trench warfare was also used in the American Civil War during the Siege of Petersburg, and the Russo-Japanese War during the Siege of Port Arthur.

Tunnels can be used to undermine fortifications, to strengthen a defence by creating the possibility of ambush, and for counterattack and transferring troops out of sight of the enemy. They can be part of an extensive labyrinth.

In Afghanistan the last hideout of Al Qaeda was a cave-and-tunnel complex in Tora Bora.

Irregular warfare and the ‘fog of war’

Tunnel warfare is especially effective in irregular or asymmetric guerrilla wars, with tunnel systems sometimes allowing a small force to overcome an enemy that is superior in numbers and weaponry.

Irregular warfare is the use of violence by sub-state groups such as insurgents, terrorists and other paramilitaries to achieve power, control and legitimacy. These groups resort to unconventional methods because of a weakness in resources or capabilities.

Underground facilities are a constant in asymmetric or irregular warfare, adding to the complexity, unpredictability and uncertainty that are inevitable in armed conflict, and contributing to the ‘fog of war’.

ISIS tunnel systems

The Islamic State created tunnel systems underneath many Iraqi cities and villages. They were essential to its strategy, enabling them to move stealthily, strike quickly and escape capture. During the Battle of Mosul (2016-17), Iraqi troops and Kurdish Peshmerga forces found that the road into the city had been honeycombed with tunnels, many of them booby-trapped.

The city itself also had an extensive network of tunnels and bunkers, to which fighters could retreat and from which they could send drones to collect information to identify the location of enemy troops, undetected by surveillance. The same was true of Fallujah. When under attack, ISIS fighters used the tunnels to hide from surveillance drones, artillery shells and US-led airstrikes.

To conceal their digging from drones and satellites, fighters would hide the dirt inside nearby houses. Many tunnels were lit with electric lights; in some, Iraqi forces found dormitories, flowered wallpaper and makeshift kitchens.

According to Iraqi forces, the tunnels made an already difficult offensive harder, allowing Islamic State fighters to appear seemingly out of nowhere, to creep quickly into position, then ambush advancing troops from concealed locations. “It’s like we are fighting two wars in two cities,” Colonel Falah al-Obaidi told The Washington Post. “There’s the war on the streets and there is a whole city underground where they are hiding. Now it’s hard to consider an area liberated, because though we control the surface, ISIS will appear from under the ground.”

Other tunnels were used to smuggle fighters and weapons across borders. A network of tunnels crisscrossing Iraq’s border with Syria was discovered in the Iraqi town of al-Qa’im. Security forces still face difficulties in clearing ISIS from desert areas, with locals reporting that the group’s remnants are hiding in valleys, caves and tunnels that are hard to locate with aerial surveillance. In March 2020, The Times reported that 2 US soldiers were killed in a battle to purge Islamic State fighters from a cave complex in northern Iraq and a year later, in March 2021, RAF and other coalition planes, in a 10-day mission, attacked up to 100 cave hideouts in Iraq. More recently, on December 30, 2021, Iraqi security forces killed ISIS fighters discovered in a cave, in a helicopter strike that killed 6 of them.

And members of the Syrian Democratic Forces (SDF) discovered tunnels that ISIS had used for smuggling into Iraq in rural areas around al-Hasakeh and Deir ez-Zor.

In Afghanistan, too, caves and tunnels have been used by ISIS as a “sanctuary” from which they have fought an irregular war against US and Afghan forces.

Despite security forces and militia groups discovering and destroying many tunnels and caves, Islamic State groups remain active in Iraq, Afghanistan and Syria. In 2021 terrorist attacks claimed over 600 civilian lives in Iraq alone, including 70 children, according to the annual report by Iraq Body Count. This demonstrates the difficulty in countering irregular warfare tactics, both on the ground and underground.

ISIS revival

As the West watched the developing crisis in Afghanistan in 2021, ISIS was regrouping in Iraq. While the Pentagon reported in December 2021 that the US had ended its combat mission in Iraq, ISIS, as French monthly Le Monde Diplomatique put it, was “filling the political vacuum left by two decades of conflict in Iraq’s most unstable region”. Its operations are mainly in the north, with as many as 1,200 active members and up to 20 attacks a week, targeting Iraqi security forces, oil and electricity infrastructure, and also civilians.

With numerous attacks in the past year, both in Iraq and in Afghanistan, ISIS seems to have no trouble recruiting fighters, especially those who are poor, disadvantaged and lacking both sufficient means for survival and hope. As Paul Rogers wrote, “Recruiting from the margins appears to be almost as easy now as 20 years ago, even with hundreds of thousands of people killed and maimed, and millions more displaced.”

While regular wars have beginnings and ends, irregular ones can be endless. The war on terror is becoming a series of conflicts without end. It seems that, in this case at least, more fundamental solutions, rather than warfare and the use of force, are needed as responses to insurgency, terrorism and extremism.