Why isn't any unprovoked act of aggression considered a war crime?

Question

I'm struggling to understand the notion of "war crimes". It refers to acts within the context of a war that are considered especially heinous and outside the normal parameters of armed conflict (the Geneva Conventions).

But why is initiating the war itself considered acceptable in the first place? E.g. in the current situation, why do we only condemn Putin of committing war crimes when we find evidence of attacks on civilians, but not the initial act of invading Ukraine?

Obviously, if a country is attacked they need to be able to defend themselves, and we would only accuse them of war crimes if they went beyond this. But shouldn't an initial, unprovoked attack be prohibited entirely?

Answer 1

An actual judicial definition of these crimes can be found in the Rome Statute that established the International Criminal Court. The statute distinguishes 4 crimes:

• The crime of genocide;
• Crimes against humanity;
• War crimes;
• The crime of aggression.

War crimes refer to criminal conduct of war that breaks established rules of armed conflict. Punished are the soldiers performing such illegal acts, as well as any superiors who failed to properly control their troops.

That's different for the crime of aggression:

For the purpose of this Statute, “crime of aggression” means the planning, preparation, initiation or execution, by a person in a position effectively to exercise control over or to direct the political or military action of a State, of an act of aggression which, by its character, gravity and scale, constitutes a manifest violation of the Charter of the United Nations.

That is, the crime of aggression is a leadership crime. Punished are those who control the state, not the soldiers who fight the illegal war.

Put differently, soldiers are required to conduct themselves according to the rules of war, while politicians are required to conduct themselves according to the rules of international relations.

And that's one reason why the distinction between war crimes and the crime of aggression matters: Both are illegal, but the perpetrators are different.

Answer 2

Waging a war of aggression is prohibited under international law in the same way as war crimes, although it isn’t a war crime itself - these are generally defined as crimes that take place during the waging of a war, such as wilful killing, torture, the taking of hostages, and so on.

The UN Charter is pretty clear in its first article that waging a war of aggression is against international law:

The Purposes of the United Nations are:

To maintain international peace and security, and to that end: to take effective collective measures for the prevention and removal of threats to the peace, and for the suppression of acts of aggression or other breaches of the peace, and to bring about by peaceful means, and in conformity with the principles of justice and international law, adjustment or settlement of international disputes or situations which might lead to a breach of the peace;

And in Article 2:

All Members shall refrain in their international relations from the threat or use of force against the territorial integrity or political independence of any state, or in any other manner inconsistent with the Purposes of the United Nations.

In fact, the resolution passed by the emergency session of the UN at the beginning of March specifically condemned Russia for breaching this article:

2. Deplores in the strongest terms the aggression by the Russian Federation against Ukraine in violation of Article 2 (4) of the Charter;

After the passage of UN General Assembly Resolution 3314, which adopted a definition of aggression, the Rome Statute was amended in 2010 (the amendment came into force in 2017) to expand the remit of the International Criminal Court, granting it the jurisdiction over crimes of aggression, defined in Article 8 bis of the Statute as:

The planning, preparation, initiation or execution, by a person in a position effectively to exercise control over or to direct the political or military action of a State, of an act of aggression which, by its character, gravity and scale, constitutes a manifest violation of the Charter of the United Nations.

Answer 3

Others have already explained that war is in fact generally prohibited in international law. The notion of war crimes is still important because it means that even if you engage in an illicit war, you should still follow some rules. This also applies to all belligerents, not only the aggressor.

Historically, some of these ideas predate the modern notion that war is bad in general. Rules on conduct during a war (jus in bello) were codified at the end of the 19th Century (the Geneva Convention but also the Hague Conventions).

A general prohibition of war and the modern jus ad bellum came much later, after World War II and the “crime of aggression” (as an individual offense, not a breach of international law by a state) is very recent.

Answer 4

But shouldn't an initial, unprovoked attack be prohibited entirely?

It is prohibited (in theory); it's just called something other than a war crime, namely

A crime of aggression or crime against peace is the planning, initiation, or execution of a large-scale and serious act of aggression using state military force. The definition and scope of the crime is controversial. The Rome Statute contains an exhaustive list of acts of aggression that can give rise to individual criminal responsibility, which include invasion, military occupation, annexation by the use of force, bombardment, and military blockade of ports. Aggression is generally a leadership crime that can only be committed by those with the power to shape a state's policy of aggression, rather than those who carry it out. [...]

Aggression is one of the core crimes in international criminal law, alongside genocide, crimes against humanity, and war crimes. In 1946, the International Military Tribunal ruled that aggression was "the supreme international crime" because "it contains within itself the accumulated evil of the whole".

However, that Wikipedia article also notes that "No one has been prosecuted for [the crime of] aggression either before or since the 1940s."