Is the conflict in Ukraine a war? This question has been raised in recent reports about a Russian invasion in Ukraine on the Caspian Sea. The USA and other NATO powers call it an “incursion”; the Baltic states and Ian Bremmer, the president of the Eurasia Group, call it war.

Using that word to describe the Ukraine crisis carries obvious political weight. Until now, it was unclear if, and to what extent, Russia was arming and supporting Ukrainian rebels. The recent uptick in the use of war vocabulary shows that international observers increasingly view this as an inter-state conflict.

The problem is that “war” is not specifically defined in modern international law after 1945 and international instruments from before 1945 do not offer clear criteria for what war is either.

Nothing to declare?

Up until 1945, it was widespread practice between states to formally declare war which may have helped somewhat to determine when a state of war existed. This tradition goes back to antiquity, and also has roots in medieval chivalry.

The 1907 Hague Convention (III) Relative to the Opening of Hostilities, itself triggered by the undeclared war between Russia and Japan in 1904, states:

The contracting Powers recognise that hostilities between themselves must not commence without previous and explicit warning, in the form either of a declaration of war, giving reasons, or of an ultimatum with conditional declaration of war.

The list of declarations of war in the first half of the 20th century is a long one: most obvious are the declaration of war by Austria-Hungary on Serbia on July 28 1914, marking the beginning of World War I, and the declaration of war on Germany by France and the UK (and India, Australia and New Zealand as part of the British Empire) on September 3 1939.

But there are also numerous historic examples to the contrary: the German invasion of Poland on September 1 1939, or Japan’s surprise attack on the US military base in Pearl Harbour on December 7 1941.

Since the end of World War II, most military engagements have not been formally declared wars (the Vietnam War being the most famous example). Again, there are exceptions, such as Sudan’s declaration of war against South Sudan in 2012. Still, the overwhelming majority of recent armed engagements have not been heralded by formal declarations, which have largely fallen out of practice in the post-1945 order.

This change has little to do with the strategic advantage of a “surprise” war, and more to do with a huge shift in the nature of modern conflict.

When push comes to shove

Wars between states are now greatly outnumbered by internal conflicts between states and non-state factions or groups, such as insurgents. Because these conflicts are by definition less clear and often intensify gradually, formal declarations of war are much less common.

But more crucially, international law underwent a fundamental shift after 1945 that left little room for conducting lawful wars; international law now prohibits all interstate uses of armed force, whether they are on the scale of war or not. War has essentially become entirely unlawful, and formal declarations of it are therefore highly unlikely, since they would only risk legal sanction.

The prohibition of the use of force is considered absolute and non-derogable. It follows from both Article 2(4) of the UN Charter and customary international law.

As far as Ukraine is concerned, this means if Russia is deemed to be using force against Ukraine, that will constitute a violation of international law. That threshold could be met directly if Russian soldiers are proven to have deliberately crossed the border into Ukraine and engage in armed activities there, or more indirectly if the pro-Russian rebels’ acts can be attributed to Russia in a legal sense – that is, if it can be proven that Russia “controls” their operations to a certain degree.

There is legal precedent here: in the case of Nicaragua v USA in 1986, the International Court of Justice held that the USA violated the prohibition of the use of force when it supported the contras fighting the Nicaraguan government by arming and training them (merely funding them was held not to be sufficient).

Exceptions

There are only two narrow exceptions to the prohibition of the use of armed force, or in other words, lawful uses of armed force: self-defence, and UN Security Council authorisation under Chapter VII of the UN Charter.

If a state wants to resort to self-defence, reflected in Article 51 of the UN Charter (or if it wants other states to assist it), an “armed attack” must have occurred. This is not just any use of force; according to the International Court of Justice’s judgement in its Nicaragua v USA judgment, only the most grave forms of use of force of a certain scale and effect, which go beyond “mere frontier incidents”, amount to armed attacks.

So far as the word “war” relates to a more intense military engagement, it can be said to be of some relevance for the threshold of a right to self-defence. A mere “border incursion” by Russia into Ukraine would not reach the level of scale and intensity of an armed attack.

The other exception, a UN Security Council-authorised military response to an unlawful use of force, has a lower threshold: the Security Council must simply establish that a threat to or breach of the peace has occurred and authorize action. But any Security Council resolution based on those facts is of course subject to the veto power of the five permanent members. As Russia is one of them, a UN Security Council response seems virtually excluded.

No loose ends

International law doesn’t just regulate the question of whether it is lawful to go to war; once a conflict begins (whether lawfully or unlawfully), another, specialised part of international law, international humanitarian law, also prohibits certain conduct and means of warfare, such as attacks on civilians or the use of chemical weapons. International humanitarian law has been much developed thanks to by the International Committee of the Red Cross.

International humanitarian law refers to war in various contexts, but its application does not depend on whether there is a war, formally declared or not. It simply applies in the case of an “armed conflict” – which may or may not have the scale and intensity of a war. This is partly in order to avoid debate about whether a war exists in a given situation which would hinder and to allow it to fulfil its function: to protect.

War is also regulated by criminal law. Violations of international humanitarian law may lead to criminal responsibility under the national laws of states, which are obliged by international law to sanction such crimes in their national criminal laws. The most severe crimes – war crimes, crimes against humanity and genocide – are also punishable at international level under the Rome Statute of the International Criminal Court.

Finally, in the future, violations of the prohibition of the use of force may make perpetrators culpable for the crime of aggression if the relevant provisions of the Rome Statute enter into force. They may already be punishable under the criminal laws of individual states.

This array of different concepts and laws might seem complicated and confusing, and the everyday clarity of the term “war” might seem like an easy alternative. But this detailed and nuanced set of regulations is in place for good reasons. It not only comprehensively prohibits the use of force in the first place, but it also helps guide international responses to illegal uses of force and to prevent escalation of force.

Calling war by its common name has volatile political and emotive consequences. In heated situations like Ukraine’s, perhaps it’s best to stick to the “cooler” terminology of current international law – and avoid carelessly escalating our rhetoric, at the risk of unintended consequences.