In the 15 years leading up to its October, 2023 terrorist attack, Hamas had been preparing to drag Israel into an underground war it couldn’t win. During that timespan, Hamas spent over a billion dollars building a 450-mile long network of tunnels that span an area roughly twice the size of Washington D.C. The Israeli Defense Force nicknamed the network of tunnels the “Gaza Metro” and the IDF’s attempts to eliminate the threat posed by this network provides a glimpse into the future of war.

Israel knew the Hamas tunnels existed, but they were unprepared for what was revealed to them when the conflict began. What was once a primitive system had grown to become what the IDF described as an “underground city” with bunkers, command centers, living quarters, weapons storage depots, medical clinics, and even data centers. There were over 5,000 shafts leading from the surface to the tunnels and the overall  network was roughly double the length Israeli intelligence had believed when the conflict started.

But it was how the design of the network had advanced that was the most shocking. Some tunnels contained three levels, which the IDF had never seen before, and were buried up to 200 meters deep, making them undetectable to Ground Penetration Radar. The tunnels contained expensive blast doors, air conditioning, oxygen supply, and sophisticated electronic and communications infrastructure. These tunnels were built underneath housing, hospitals, mosques, and schools, to turn civilians into human shields and all of Gaza into a fortress. Virtually every tunnel was booby trapped.

In early 2023, senior Israeli military officials had concluded that Hamas’s tunnels would not be a factor in future wars due to the IDF’s technological overmatch. But they were mistaken. The tunnels were Hamas’s solution to Israeli technological supremacy. The Israeli military has an insurmountable advantage when fighting on the surface, where its intelligence, surveillance, and reconnaissance systems combine to form an all-seeing eye on Gaza and its total aerial dominance allows it to hit any target anywhere in the territory.

But underground, the advantage goes towards the defenders, with attackers being forced to fight blind in a terrifying environment. The equipment Israel and Western armies rely upon does not function underground.  Communications relying on line-of-sight or satellite signals fail as do navigation tools such as GPS and compasses. Drones often lose signal connectivity, and night vision goggles do not work because there is no ambient light underground.  This is the reason the U.S. Army's Subterranean Operations manual states “Soldiers must avoid entering and fighting in a subterranean environment when possible.” 

Hamas is not alone in the adoption of its subterranean warfare strategy. There are an estimated 10,000 large-scale underground military facilities worldwide, many of which are designed to function as subterranean cities. Both Ukraine and Russia have built tunnels to infiltrate enemy positions undetected and rebel forces in Syria, including ISIS, the Islamic Front, and Al-Nusra, built tunnels to attack Syrian Armed Forces. Kurdish fighters built a network of tunnels to recapture territory seized by Turkish-backed forces.

In the future, U.S. soldiers––like Israeli soldiers–– may not have a choice about whether to fight underground. The war in Ukraine has shown that fighting on the surface now means entering a drone-filled kill zone; the Ukrainian military estimates that drones are responsible for approximately 70-80% of its casualties. In modern war, anyone above ground is a target that can be killed.

Just as the 1936 Spanish Civil War served as the testing ground for the military tactics and technologies that were used in World War II, the wars in Ukraine and Israel are showing us what the future of war looks like. And it is brutal.

Tunnel warfare dates back at least 3,000 years; archaeologists have discovered Assyrian reliefs from the 9th century BC  depicting military engineering units (belonging to Sargon of Akkad) building tunnels to penetrate the walls of enemy cities. 

The tactic was taken to a new level during the period of the Roman empire. Jewish insurgents built 450 tunnels, many dug into mountains, to stage hit-and-run attacks on Roman legionnaires during the Great Jewish Revolt, which lasted from 66 AD to 70 AD. 

The Romans used tunnels to penetrate fortifications when they placed enemy cities under siege. Romes’s enemies adapted to this threat in ways that illustrate the cat and mouse nature of underground warfare. The Roman historian Polybius described the first use of acoustics in warfare during a siege at the Greek city of Ambracia in 189 BC. The Greeks noticed the earth moving outside their walls and in a panic dug a trench parallel to the city’s walls where it placed a series of brazen vessels which vibrated to sound and allowed the Greeks to detect and precisely attack enemy tunnels. 

The Romans developed their own counter tunnel tactics including filling enemy tunnels with poisonous gas during the defense of the Roman city of Dura-Europos in 256 AD against the forces of the Persian Empire, which was the first recorded example of chemical warfare in history.

Over time, a new innovation was brought to tunnel warfare tactics - gunpowder - which European armies used to blow up enemy fortifications. These fortifications were called saps, which led to combat engineers being dubbed sappers, a label that continues to be used today.

This technique could lead to disastrous consequences when things went wrong. Union soldiers built tunnels under Confederate lines during the Siege of Petersburg and filled them with so many barrels of gunpowder that when detonated,  the explosion created a 170-foot long crater that is still visible today. The Confederates were too stunned to direct any fire at the Union soldiers immediately after the explosions. But soon, they discovered their adversaries were trapped in a hole and what became known as the Battle of the Crater began in earnest. Confederate soldiers surrounded the crater and fired artillery and rifle shells for an hour in what one soldier would later describe as a turkey shoot.

With the advent of mechanized warfare in World War I and the introduction of tanks, massed artillery, and military planes to the battlefield, military planners realized that ground forces were too exposed to danger and began considering new possibilities, including building large underground complexes.  Most famously, France built a series of underground fortifications, including an underground railway, across its border known as the Maginot Line prior to World War II.

In the Pacific theatre during World War II, Chinese guerillas dug miles of tunnels between villages that were designed to enable them to attack Japanese forces from behind. The Japanese began filling the tunnels with water and poison gas but later adopted similar tactics of their own. On the islands of Peleliu and Iwo Jima, the Japanese built extensive tunnel complexes and even turned a mountain, Mount Suribachi, into an underground lair on Iwo Jima. The Marine Corps fought an intense battle, using grenades and flamethrowers to clear the tunnels, and suffered casualty rates twice as high compared to other island battles and it took months after Iwo Jiwa had fallen for all tunnels to be fully cleared. 

The U.S. would again find itself facing the dangers of underground warfare in Vietnam, where Communist insurgents had spent 20 years building 200 miles tunnels across the country. The U.S. military tried to destroy tunnels from the air––dropping 750 pound bombs on tunnel entrances––but aerial attacks proved ineffective. The U.S. Army next tried to “smoke out” the Viet Cong using tear gas and smoke bombs. This too proved futile, so the Army began recruiting wiry soldiers who were 5’5” or shorter, to go into tunnels. The narrow confines of the tunnels meant soldiers would enter them armed only with a flashlight, knife, handgun, and their own almost unimaginable mental toughness. These men became known as “tunnel rats.”

Inside the pitch black-tunnels, tunnel rats were confronted with false walls designed to collapse on enemy intruders and booby traps, including punji stake traps, and tripwires connected to boxes filled with venomous snakes or scorpions. Fighting the enemy often meant hand-to-hand combat. Almost all of the around 100 Army tunnel rats did not leave Vietnam alive.

The U.S. would next encounter tunnel warfare in Iraq. During the Second Battle of Fallujah, the Marine Corp became engaged in the most intense urban combat the U.S. military has faced since the battle of Hue City in Vietnam in 1968. Enemy fighters spent months building underground tunnels between buildings to enable surprise attacks on incoming American fighters. The Marines were forced to go house to house clearing the city of enemy fighters.

In the aftermath of the Battle of Fallujah,  Marine Corps Major Doug Zembiec would state his men had “fought like lions” throughout the battle. But victory came at a high cost, with 95 U.S. fatalities versus around 1,500 enemy fighters killed and 2,000 captured. The Battle was the bloodiest combat operation U.S. forces have been engaged in for decades. The ability of subterranean urban combat to level the playing field for insurgents facing a technologically superior force explains its enduring appeal. 

All Western militaries are built around the concept of maneuver warfare, which the U.S. Marine Corps describes as a "warfighting philosophy that seeks to shatter the enemy's cohesion through a variety of rapid, focused, and unexpected actions which create a turbulent and rapidly deteriorating situation with which the enemy cannot cope.”

Maneuver warfare entails the close integration of infantry, armor, and air power, which is enabled by GPS navigation, encrypted radio communications, aerial surveillance, and other advanced systems. Subterranean warfare mitigates these advantages, with many electronic systems rendered inoperable underground. If warfighting goes underground, the entire Western philosophy of war may have to change.

Israel’s subterranean warfare experience points toward how the Western way of war may evolve in the future. The tip of the spear for the IDF when fighting in this new underground domain is an elite commando unit known as the Yahalom unit, which operates under the IDFs military engineering corp. First formed in 1995, the unit has doubled in size to an undisclosed number over the past 20 years.

The IDF has been preparing to fight a large-scale underground conflict against Hamas for years. Its battle plan following the October 2023 attack was to first target known underground strongholds with bunker busting bombs before beginning a ground assault on Gaza. During the ground assault, Yahalom broke into squad-sized units to fan out across a wide area and enable a fast operational tempo. 

Regular IDF forces used what they called the “triangle system” where they looked for locations within areas that contained schools, mosques, and hospitals, which are attractive shields to deter IDF air attacks. The IDF also used a classified sensor technology code-named Power Number as well as Ground Penetrating Radar to detect tunnels. Once tunnels were identified, Yahalom was called in to investigate. This process entailed using robots, drones, and specially-trained dogs equipped with cameras from a military dog unit called Oktez.

The vulnerability of even elite units when fighting underground was quickly confirmed when 5 members of Yahalom were killed by a booby trap hidden inside tunnel walls in November 2023. To avoid risks to its soldiers, the IDF began considering novel ideas to clear out the tunnels. One such idea was dubbed the Atlantis Project, which end tailed using industrial water pumps to literally flush fighters out. This tactic proved both time consuming and ineffective because of porous soil conditions and the drainage systems installed in the tunnels as a countermeasure by Hamas.

As the exploration of the tunnels continued, the IDF began to realize the tunnels functioned as a “system of systems” with a unique architecture depending on the purpose of the tunnels. Some tunnels were connected and had key operational roles while others were small disconnected tunnels designed to enable guerilla-style hit-and-run attacks.  Through detective work, the IDF was able to develop a map of the tunnel systems, with different tactics employed to address the different types of tunnels.

But a core problem remained; Hamas was able to control the initiative of most battles simply by using booby traps to collapse tunnels as they retreated. This frustrated IDF commanders and prompted Brigadier General Dan Goldfus, who was formerly a member of Shayetet 13, the IDF’s version of  U.S. Navy Seals, to develop a strategy to enable Israeli forces to retake the initiative. 

His plan revolved around sending commando units and combat engineers armed with specialized equipment to attack uncleared tunnels while at the same time IDF forces were closing in on enemy locations above ground and cutting off any above ground escape points. It was the first time in history a military had engaged in maneuver warfare simultaneously above ground and below ground in a dense urban environment. The IDF regained the initiative and dictated terms to its enemy.

But to achieve a lasting victory, the IDF needed to find a way to demolish the Hamas tunnel systems. It has used a variety of methods to accomplish this task, including drilling holes into tunnels at set intervals and injecting liquid TNT, using bunker busting bombs, and simply lining tunnels with explosives, but the reality is this process could take years to complete.

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Since the founding of Israel in 1948, the country has had to exist continuously on the precipice of war. It has fought 9 major conflicts and countless smaller-scale conflicts such as the First and Second Intifadas. Because the IDF is so frequently engaged in war and armed with primarily American-made weapons, U.S. military strategists have long studied Israel’s warfighting experience.

The Egyptian and Syrian armies success using new Soviet weapons and tactics during the 1973 Yom Kippur War resulted in the U.S. military making radical changes to its doctrine.  U.S. military strategists would develop a foundational doctrine known as “AirLand Battle” where land forces would act as an aggressively “maneuvering defense” against enemy forces while an air attack decimated the supply lines to these front-line enemy forces. To utilize this new style of war required new types of weapons, and this led to the development of the Patriot Missile and HIMARS systems used to great effect by Ukraine as well as the M1 Abrams tank, Bradley Infantry Fighting Vehicle, and the stealth Lockheed F-117 Nighthawk.

The U.S. military is cooperating with the IDF to learn from its experience and how its tactics have evolved. But the U.S. was already focused on the problem of underground warfare; the Gaza war just made planning for one much more urgent. In 2018, the U.S. spent close to $600 million building underground training complexes and equipping and training 26 of its 31 active combat brigades in how to fight in such an environment. 

At around the same time, DARPA initiated the Subterranean Challenge or SubT where teams competed to develop autonomous robots capable of mapping, navigating, and searching underground spaces. The competition was held at the Louisville Mega Cavern in 2021.

The U.S. is now testing robots optimized for underground warfare in cooperation with South Korean forces. It has also developed specialized units, such as the U.S. Army Special Forces “Hard Target Defeat” companies, who are focused on fighting in primarily underground environments where enemies have placed command and control nodes, weapons of mass destruction, and vital infrastructure.

A major reason for the U.S. military’s recent focus on underground warfare is the increased urbanization of the planet. There are currently 33 cities with 10 million or more people; by 2050 this number is expected to reach 67, primarily in Africa and Asia. If a conflict arises in these regions, underground warfare becomes almost an inevitability because the subsurface offers fighters a way to neutralize technological overmatch and exploit urban complexity.

While the developed world shrinks in population, sub-saharan Africa is expected to double its population and add over 1 billion more people by 2050. Studies have shown that 94% of the current sub-saharan Africa population lack the basic skills needed to compete in the global economy and the region is responsible for 59% of global terrorism-related deaths.

While many in Washington are preoccupied with China, there is an equally if not more plausible scenario where the U.S. military is forced to intervene in chaotic failed African or Asian states against an enemy entrenched in megacities. And if this day comes, the U.S. will almost certainly be forced to fight against its enemies underground and in the dark.