Egypt reportedly gassed Hamas tunnelers on Monday leading to two deaths and several injured as part of ongoing efforts to eliminate Hamas’ cross-border tunnels between it and Gaza.

Although deadly gas has been banned as a method of war even against combatants for a century there were no global outcries against Egypt.

In contrast to Egypt, Israel has declined to use gas or flooding tunnels with sewage water.

Recently, Israel has also favored filling tunnels with cement, which may be among the least lethal methods of neutralizing tunnels. Were Israel to use Egyptian methods, it would presumably face a wave of global criticism as it does nearly any time it uses force.

What principles of international law apply to tunnel warfare, how does Israel try to clear tunnels without violating the law and why the double-standard?

First, Egypt is not the only one to use gas against adversaries in tunnels.

As IDC Herzliya Professor and underground warfare expert Daphne Richemond-Barak has pointed out in her book Underground Warfare, US forces pumped tear gas into the Vietcong tunnel complexes during the Vietnam war.

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Technically, tear gas does not have the same absolute ban that deadly gas does, but in tunnels it sometimes caused suffocation, raising questions about its legality.

Likewise, the USSRs pumped unidentified gases into sections of the Karez irrigation system as part of the Soviet-Afghan war.

Part of the variety of tactics used against tunnels depends on different eras and countries that care more or less about international law.

But part of it is a general confusion about what legal framework tunnels work into.

Richemond-Barak says that core international humanitarian law principles like distinction and proportionality still apply to tunnel warfare even if tunnels have unique features.

In other words, whether mapping, neutralizing or eliminating tunnels, militaries must be able to categorize the tunnel as a legitimate target, the damage to civilians should not be “excessive” relative to the military advantage anticipated from the attack, and all feasible precautions should be taken to evacuate civilians.

Egypt has also reportedly used flooding against Hamas and its tunnels.

Richemond-Barak notes that flooding , if it creates an entrapment situation, is likely to raise concerns of “no quarter” or no survivors style warfare which violates Article 40 of Additional Protocol I to the Geneva Conventions.

In recent months, Israel has recently preferred blocking tunnels with a cement-like substance or detonating them after clearing them.

Cement can neutralize a tunnel rapidly, but it does not necessarily eliminate the threat altogether.

Richemond-Barak points out that, “It is also a costly method that requires expertise, as cement hardens quickly and must be manipulated carefully,” – a reason why countries less concerned about law might avoid using it.

Moreover, while “no significant legal issues arise when cement is only used to seal off” one side of a tunnel, there could be international law problems “if all entrances and exits are sealed off while people are present inside the tunnel.”

Israel has reportedly mostly used robots and other methods to ensure that adversaries are not in tunnels when it neutralizes them with cement, as highlighted in the recent operation against Hezbollah tunnels.

One reason is that, unlike many other methods, as Richemond-Barak states, “cement does not directly or indirectly affect those living in the vicinity of the tunnel.”

This is not to say that Israel has not killed Gazans inside tunnels. In October 2017, Israel destroyed a tunnel with explosives killing several Islamic Jihad operatives.

However, in that case Israel at least claimed that the operatives were not killed by the explosives, but when they entered the tunnel afterwards.

Why is there so much concern about killing Hamas operatives or other adversaries in tunnels?

Aren’t tunnels just another weapon of war and operatives inside them can be presumed to be combatants who might at any moment try to stage a surprise attack?

Wouldn’t that justify neutralizing a tunnel, including killing anyone inside, no different than firing on Hamas operatives in a vehicle or a building?

The answer is complex and could depend on the terrain and how close the tunnel is to civilian areas.

If the tunnel is close to civilians’ areas, as Hamas and Hezbollah often intentionally build them, then the IDF is confronted by the new unique challenge that civilians may hide in them during hostilities and it may be nearly impossible for the IDF to see they are there.

Also, while tunnels have been used as tools for ambushes in wartime, often Hamas has used them to smuggle commercial products for sale.

If there is no general war and there has not been since 2014, it is not clear that Israel has the right to kill a Hamas operative in a tunnel who presents no threat at that moment, especially if the IDF has the ability to evacuate him without risking its soldiers while still achieving the mission of sealing the tunnel.

If Hamas operatives in a tunnel are getting ready to attack or present a threat, they could be attacked like any another Hamas fighter.

Why there is a double-standard for criticism of Israel when it kills Gazans versus Egypt is likely less related to tunnel warfare than to the general trend of global human rights groups policing Israel’s use of force more carefully than many other countries.

As multiple experts have pointed out, the UNHRC report on the 2014 Gaza War had a whole section on the psychological harm from Israeli drones on Palestinians, but practically entirely ignored the psychological harm on Israelis from the tunnels and other Hamas threats.

The UNHRC could not even clearly declare that the tunnels were used for warfare despite their being used to ambush IDF soldiers in several instances.

Both to maintain its own ethics standards and to preempt the double-standard of critics, the IDF is likely to continue to use non-lethal methods of neutralizing tunnels even as Egypt may continue to sue deadly methods.