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:
- 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.