Early on 20 June 2019 Iran shot down a US unmanned aerial vehicle over the Persian Gulf; a drone that was allegedly infringing on Iranian territory. According to Iranian authorities, the drone did not respond to warnings and was destroyed just after 4am local time, a few miles off Iran’s coast. The US denies having received any warnings and claims the drone was downed 21-miles offshore in an unprovoked attack.

Although Iran was the first country to weaponise drones in the mid-1980s, the US is now by far the largest drone user; since the country first used drones in 2001, its deployment has been steadily increasing. Six times as many drone strikes were launched during the first term of Barack Obama’s presidency than in both of George W Bush’s terms combined (Obama’s tenure was aptly nicknamed the “drone presidency”). Under the Trump administration, strikes have quadrupled to an average of one every 1.25 days.

Traditional warfare involves state-to-state combat between trained soldiers. Since 1949, a set of internationally agreed rules, known as the Geneva Conventions, have been observed in efforts to protect civilians and wounded and captured soldiers. But drones are part of the fight in a new kind of war: the war on terror. Its combatants neither serve for nation states nor wear uniforms; fighting is rarely confined to a designated warzone. The rise of military drone use raises new questions about the ethics of war.

In 2016, Obama attempted to close these ethical gaps. The former president signed an executive order to minimise civilian casualties in all uses of force. Later that year he published presidential policy guidance on “direct action against terrorists”, which laid out a process for authorising drone targets in line with the principles of Just War Theory, a tradition of military ethics that determines which actions are acceptable when waging war. The four key principles are humanity, necessity, proportionality, and discrimination.

The principle of humanity requires avoiding unnecessary harm to civilians. The fundamental rationale for drone use relies on this principle: drone capacity for precise targeting can save civilian lives. US Air Force General T Michael Moseley calls this the “true hunter-killer role” of drones. Given that there were, for example, two million civilian deaths in the Korean War alone, the precision of drone strikes offers a persuasive moral argument in favour of their use

Yet it is hard to determine how many people have been killed by drones in the war on terror. Information about US drone strikes remained mostly classified until the Obama administration was obliged by court order to release data, which showed that roughly 2,500 terrorists, and somewhere between 64 and 116 civilians, had died at the hand of drone strikes. But some estimates put the number of al-Qaeda or Taliban leaders killed at only 2 per cent, a discrepancy that suggests drone strikes are not always as precise as claimed. If so, can they ever uphold the principle of humanity?

The principle of distinction stipulates that military operatives must have strong reason to believe a target is an enemy combatant. Obama took the decision about combatant status away from the CIA, and his policy guidance established a process for authorising strikes that required certain conditions were met, including the near-certainty of target presence and civilian safety, the infeasibility of target capture or alternative action, and the unwillingness or inability of host-country action.

With that said, the principle of distinction is hard to follow in the war on terror. It’s almost impossible to identify combatants who don’t wear a uniform and blend in with the civilian population. One attempt to solve this issue is through a “signature strike” that identifies combatants using patterns of behaviour. People in regular contact or close proximity to an identified combatant are assigned combatant status. But as a senior US state department source put it: “when the CIA see three guys doing jumping jacks, they assume it’s a terrorist training camp.” In other words, drone strikes don’t requirements of distinction in the war-on-terror.

The principle of necessity requires that targets have definite military value. Drones clearly have “military value”; they cross difficult terrain and reach remote areas more easily than expensive ground assaults, and they keep soldiers out of battlefields where their lives are at risk. The “military value” of drones seems intrinsic to their very use. Of course, if drone use always has military value, then the principle of necessity is redundant for the ethical assessment of drone use.

Finally, the principle of proportionality requires that military operatives use a reasonable amount of force. After Iran downed the US drone in June, Trump tweeted that “Iran made a very big mistake” and ordered a retaliatory strike. He later aborted the mission; it would have been disproportionate to risk 150 lives in responding to the loss of a single drone. But there’s a sense in which US drone deployment is always disproportionate. The countries and people it targets are radically under-equipped in comparison to America’s military might. This raises questions about whether drone use can ever be considered a “reasonable” or “proportionate” use of force, in general.

Ultimately, drone warfare departs from Just War Theory. But this isn’t the only ethical issue it raises. The scope and legitimacy of drone use are also problematic. In international humanitarian law, the UN charter doesn’t support countries flying armed drones when they are not at war, and bans firing missiles against rogue forces like ISIS and Al Qaeda without local government agreement. The US has departed from this charter – its Iraq Resolution, which was issued after 9/11, authorized military action on foreign soil outside war zones. Obama’s policy directive, meanwhile, allows for a “global” war on terror – effectively treating the whole planet as a battlefield for extrajudicial executions.

Crucially, Article 51 of the UN Charter specifies the right to self-defence. While 9/11 may have justified drone strikes against al Qaeda, the 40 strikes in Yemen in March 2017 that killed hundreds of civilians cannot possibly constitute self-defence against imminent harm. The US is now proposing that “self-defence” be redefined to include anticipated terrorism as “imminent.” This is a contradictory and dangerous move: defence is inherently responsive, and has never before been taken to justify action prior to an actual attack. While the UN charter requires countries to immediately report defensive action, and allows such action only as a measure until the security council can intervene, there is no evidence that the US has respected either qualification when deploying drones.

The effects of military drones go beyond the casualties they create. Pilots are required to confirm deaths and identities – meaning they witness, close-up, the impact of their work. It’s no surprise, therefore, that drone pilots experience a far greater degree of post-traumatic stress disorder than other “distance warriors” – stress that may be amplified by the isolated experience of drone operatives, who can’t discuss their work with family or friends.

In the end, US drone deployment in the war on terror exploits the gaps in international humanitarian law and shows little regard for the well-being of soldiers. Though tensions between the US and Iran may mark the introduction of drones into more traditional state-to-state warfare, drone use may also render short-lived the twentieth century hope that war can be regulated. Given the recent drone attacks on Saudi oil fields, perpetual and unregulated wars may be closer than we think.

Trish Glazebrook is Professor of Philosophy at Washington State University. She is the author of Heidegger’s Philosophy of Science.

This article is part of the Agora series, a collaboration between the New Statesman and Aaron James Wendland, Professor of Philosophy at the Higher School of Economics. He tweets @ajwendland.