It was long thought of by international legal experts as "a dead letter" – a law that remains on the books, but is generally considered to be outdated. Article 4.A.6 of the Third Geneva Convention, a core text of the international laws of armed conflict, contains a provision for "inhabitants of a non-occupied territory, who on the approach of the enemy spontaneously take up arms to resist the invading forces, without having had time to form themselves into regular armed units". In other words, civilians who pick up weapons and fight back on their own against invaders – also known as a "levee en masse", or a mass mobilisation.
The levee en masse was first conceptualised during the 18th-century French Revolution, when France, at war with most of its neighbours, conscripted virtually its entire population to either fight or assist the war effort. It was, at the time, a very unpopular measure.
It became a dead letter because, since the Second World War, few states have suddenly invaded other sovereign states and, when they have, they have not been met with large numbers of civilians scrambling to fight back in the heat of the invasion. Invaders have fought militaries, militias and insurgencies, with some civilians occasionally thrown into the mix. But since the German invasion of the Greek island of Crete in 1941, when Cretan civilians used everything from hunting rifles to walking sticks to attack German paratroopers, they have not encountered a true levee en masse. War has not worked that way for a long time.
Thanks to the military strategy of Ukraine's President, Volodymyr Zelenskyy, it does now.
Within a few hours of Russia's decision to invade Ukraine, Mr Zelenskyy barred adult men from leaving Ukrainian territory and dramatically simplified the requirements to enlist in the country's armed forces, as well as a new paramilitary organisation called the Territorial Defence Forces. Most extraordinarily, however, he has sought to create a levee en masse through Twitter.
"We will give weapons to anyone who wants to defend the country," tweeted Mr Zelenskyy. The Ukrainian military has also used the social media platform to call on citizens to make Molotov cocktails, and Hanna Maliar, the deputy defence minister, used Facebook to urge civilians to throw them from their balconies.
Unlike in revolution-era France, Mr Zelenskyy's measures have proven very popular in Ukraine, where Google searches for "How to make Molotov cocktails" spiked on Friday.
All of this raises the fear, however, that as the lines start to blur between ordinary citizens and soldiers, the stage will be set for a much messier conflict. Missiles have already hit several civilian targets throughout Ukraine, and civilians have reportedly been shot in the streets. Under international law, such incidents would almost certainly constitute war crimes. The use of civilian balconies to launch homemade bombs, however, could complicate matters.
Under the Geneva Conventions, civilians who participate directly in hostilities are classed as combatants. While they engage in such hostilities, they lose the protection they might have enjoyed as civilians, though they gain a new protection in that, if captured, they are immune to prosecution as criminals or terrorists and must instead be treated as prisoners of war (provided they have themselves abided by the laws of war when fighting). That is to say, even foreign aggressors and occupiers, according to the laws of war, enjoy a limited right to self-defence.
But it is important to emphasise the word "limited".
There is a long-running debate within international humanitarian law over how to treat civilians who participate directly in hostilities. One view is that those who take up arms become members of an armed group, and are targetable permanently, wherever they are or whatever they are doing, in the same way any soldier is during wartime. The other view is that they are targetable only insofar as they have a "continuous combat function" – i.e. "for the duration of each specific act amounting to direct participation in hostilities", as the Red Cross puts it.
For the past two decades, the chief proponent of the first view was the US, which has sought to target Taliban and Al Qaeda militants, as well as any part-time militants, so to speak, even when they are off the clock. The results have been catastrophic for civilians in regions where these militants have operated – evidenced by the huge number of civilians who have been killed in drone strikes on public gatherings where militants were thought to be present.
By repeatedly seeking to justify the membership-based view on the international stage, the US may have unwittingly given legal ammunition to those who may wish to justify targeting civilian areas in future conflicts, particularly if they contain people involved in the war.
Nonetheless, the UN, Red Cross and most legal scholars disagree with the membership-based approach, arguing that civilian combatants are only targetable while actually fighting. The rationale is to limit, to the greatest extent possible, the instances in which civilians can be targeted, so as not to allow the targeting of civilians to spin out of control.
For armies pitted against civilian combatants, this creates a significant burden. In practice, it will be difficult to draw lines between civilians making bombs under instructions from the defence ministry, citizen volunteers for a loosely organised territorial defence force, and enlisted service members. One problem for civilian combatants, however, is that the longer their participation in a war drags on, the stronger the opposing army's case becomes that they are permanent members of an enemy force, and are generally targetable.
When that happens, the debate shifts from membership to proportionality – an issue that has dogged the Israeli military, for example, in its own ongoing conflict with militants using civilian buildings in Palestinian territory. The Israeli Defence Forces have long justified airstrikes on civilian buildings in Gaza by claiming they are used as bases by the militant group Hamas. While their function as bases may indeed make them targetable, actually targeting them, according to most legal experts, runs afoul of another principle of international law, which is that the military objectives achieved by such attacks must be proportional to the risks posed to civilian life and property. In other words, targeting an entire residential building to eliminate a relatively small threat is illegal.
Targeting civilian combatants without risking large numbers of other civilians and their assets is very difficult, and laws on proportionality are part of what make the concept of a levee en masse such a potent force to be reckoned with. They help to create a class of fighters who can pose a real threat, but are hard to target and even harder to target legally. If this seems like it privileges civilian combatants over regular fighters, that is because it is designed that way, to provide a temporary, but big tactical advantage to civilian combatants who have found the battlefield brought to them and, out of sheer desperation, are forced to respond.
Nonetheless, the Geneva Conventions require even these combatants to "respect the laws and customs of war". It is worth noting that during the German invasion of Crete, several Cretan civilians were accused of war crimes. Although they were never prosecuted, that does not mean civilians who find themselves in similar situations could not be today. The chaotic situation in Ukraine breeds opportunities for all manner of people to take up arms with the blessing of the state, and as it continues the state may have diminishing control over how all of them behave.
The obvious question on many people's minds will, of course, be whether any attempt to establish legal clarity during such a desperate moment in Ukraine's sovereign history is warranted. They may cite Cicero, the Roman lawyer who wrote more than 2,000 years ago that "laws are silent amid the clash of arms". But in Ukraine, the laws are, in fact, speaking very loudly, if not clearly. Violations of international law were cited by Russia as a pretext for its invasion, and have been cited by Ukraine in its defence. It is important that, when the dust settles, the world builds a clear picture of where the law stands, and to what extent everyone involved operated within it.