Human shields have been making headlines for some time. Before the recent fray between the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) and Iraqi army in Fallujah, the United Press International released an article entitled "Iraqi forces halt Fallujah advance amid fears for 50,000 human shields".

Indeed, not a day has passed in the past several months without an array of newspapers mentioning human shields in different theatres of violence: Fom Syria, where ISIL fighters fled Manbij in convoys apparently using human shields; through Kashmir, where "army and police used civilians as human shields in operations against militants"; to Ukraine, where pro-Russian separatists were accused of using international observers as shields.

Moreover, the phrase human shields is not only used to describe the use of civilians in the midst of war, but to depict civilians in protests, from Ferguson in the United States, to Zimbabwe and Ethiopia.

Liberal democratic states are not the only ones who are warning the world of the increasing use of human shields; rather authoritarian regimes as well as a variety of local and international organisations of different kinds, from the Red Cross and human rights NGOs to the United Nations, are invoking the term.

In a recent confidential UN report, Houthi rebels were blamed for concealing "fighters and equipment in or close to civilians … with the deliberate aim of avoiding attack."

Allowing killing

Although different forms of human shielding have probably been conceptualised and mobilised since the invention of war, its quotidian use is a completely novel phenomenon. Why, one might ask, has this term suddenly become so pervasive?

Legally speaking, human shields refer to the use of civilians as defensive weapons in order to render combatants or military sites immune from attack. The idea behind the term is that civilians, who are protected under international law, should not be exploited to gain a military advantage.

Given the strategic and pervasive adoption of the phrase human shields, it seems clear that the term is not only being deployed as a descriptive expression to depict the use of civilians as weapons, but also as a kind of pre-emptive legal defence against the accusations of having killed or injured them.

While most people will undoubtedly be familiar with this definition, less known is the fact that international law not only prohibits the use of human shields but also renders it legitimate for militaries to attack areas being "protected" by human shields.

The US Air Force, for example, maintains that "lawful targets shielded with protected civilians may be attacked, and the protected civilians may be considered as collateral damage, provided the collateral damage is not excessive compared with the concrete and direct military advantage anticipated by the attack."

Along similar lines, the 2013 document on joint targeting published by the US Joint Chiefs of Staff underscores the importance of the principle of proportionality, it also notes that, "otherwise lawful targets involuntarily shielded with protected civilians may be attacked … provided that the collateral damage is not excessive compared with the concrete and direct military advantage anticipated by the attack." (PDF)

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What all this means, quite simply, is that human shields can be legally killed so long as the deployment of violence does not breach the principle of proportionality - which requires belligerents to refrain from causing damage disproportionate to the military advantage to be gained.

It now appears that police forces the world over are adopting a similar perspective as they confront protests and riots.

The motivation behind the adoption of such guidelines by domestic and international actors is clear: It allows security forces to relax the rules of engagement, while framing those who deploy shields as morally deplorable and in breach of international law.

Pre-emptive legal defence

Given the strategic and pervasive adoption of the phrase human shields, it seems clear that the term is not only being deployed as a descriptive expression to depict the use of civilians as weapons, but also as a kind of pre-emptive legal defence against the accusations of having killed or injured them.

Put differently, if any one of Fallujah's 50,000 civilians is killed during an anti-ISIL onslaught, then it is not the US-backed attacking forces that are to blame, but rather ISIL itself, which illegally and immorally used civilians as shields.

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Moreover, it increasingly appears that it is enough to claim - in advance - that the enemy is using human shields in order to warrant the killing of non-combatants.

Even though it is undeniable that many militaries and non-state armed groups do, in fact, use human shields, the potential ramifications of the mere accusation are extremely worrisome.

In other words, by claiming that the other side is using human shields, the attacking force provides itself with a pre-emptive legal defence.

To understand fully the implications of this framing it is imperative to take into account that urban areas, as Stephen Graham from Newcastle University put it, "have become the lightning conductors for our planet's political violence."

The fact that warfare currently shapes urban life in many areas around the globe means that civilians occupy and will continue to occupy the frontlines of much of the fighting.

This leaves them extremely vulnerable to being framed as human shields, since it would be enough to say in advance that the residents of a city are shields for their deaths to be legal and justified.

Insofar as this is the case, then the pre-emptive legal defence may very well be used as part of a horrifying process aimed at legalising and normalising the massive slaughtering of civilians.

Neve Gordon is a Leverhulme visiting fellow at SOAS, University of London.

Nicola Perugini is lecturer at the School of Social and Political Science, University of Edinburgh.

The views expressed in this article are the authors' own and do not necessarily reflect Al Jazeera's editorial policy.