Based on International law, to what extent can Ukraine take aggressive military action within the territory of Russia?

Question

I wonder, based on international law, if Ukraine takes aggressive military action in the territory of Russia, such as occupying some cities of Russia (I am not considering Donbas, Luhansk and Crimea etc. as the territory of Russia), will that be considered legitimate? If not, to what extent is Ukraine allowed to take aggressive military action within the territory of Russia?

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I am asking this because from the military-strategic point of view, it seems that some feint attacks from the Northeast within the territory of Russia could be helpful in attracting part of the Russian army to that direction and might make it easier for Ukraine to take back its Eastern territory currently occupied by Russia.

Answer 1

International law does not distinguish between targets in Russia or Ukraine. What matters is that the target is legitimate, i.e. a military objective.

Article 52(2) of the 1977 Additional Protocol I to the Geneva conventions states:

2. Attacks shall be limited strictly to military objectives. In so far as objects are concerned, military objectives are limited to those objects which by their nature, location, purpose or use make an effective contribution to military action and whose total or partial destruction, capture or neutralization, in the circumstances ruling at the time, offers a definite military advantage.

So a Russian military base which is somehow used to prepare an attack is legitimate target. Targeting a city which does not significantly contribute to the war directly with the intent to force Russia to divert its troops to defend that city is not legitimate.

That's the international law perspective in simple terms.

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There is, however, another angle. Much of Ukraine's military capabilities are based on weapons supplied by the Western allies which do not necessarily want their weapons used on Russian soil for fear of needless escalation.

An example of that has been reported with respect to the UK's supply of storm shadow missile, according to CNN:

Critically, the Storm Shadow has the range to strike deep into Russian-held territory in Eastern Ukraine. A Western official told CNN that the UK has received assurances from the Ukrainian government that these missiles will be used only within Ukrainian sovereign territory and not inside Russia. UK officials have made frequent public statements identifying Crimea as Ukrainian sovereign territory, describing it as “illegally annexed.”

Notably, the US has so far not provided missiles with that attack range, from the same CNN article:

The missile is “a real game changer from a range perspective,” a senior US military official told CNN and gives Kyiv a capability it has been requesting since the outset of the war. As CNN has reported, Ukraine’s current maximum range on US-provided weapons is around 49 miles.

This reluctance in providing increasingly offensive capabilities is not a constraint imposed by international law. Instead, it's about a fear for escalation and other reasons. For example, more advanced equipment (e.g. fighter jets) requires a bigger supply / maintenance chain which could require direct Western involvement by deploying maintenance crews or servicing in nearby countries which involves them more directly in the war effort.

Answer 2

Following my source, there are no restrictions that military targets would only be allowed at the own occupied territory. Of course, the sides must be at war to permit any military targets, starting from.

It is possible to imagine cases when seizing a small part of Russian territory would provide significant military advantage, like closing the important supply route or just striking into the occupied territory from the opposite side where defense lines may be less developed. Maybe even some skirmishing would qualify, to force Russia keeping defending forces there.

Russia took this possibility into consideration by developing lines of fortifications inside the own territory as well, near Belgorod (source, Reuters), exactly to oppose the attack in the direction where your arrows go.

Any such plans must be carefully weighted as they would provide a boost for Putin's propaganda, even if I do not know if they can add much to that they have already said. But generally this should be a valid and understandable military target as long the war is ongoing, and unlikely to be seen as a war crime given other conditions are met (not too many civilians killed, etc).

Later of course these parts would not be seen by the world as legally part of Ukraine.

Answer 3

TL;DR: Based on international law, if Ukraine takes aggressive military action in the territory of Russia, including, but not limited to, occupying some or all cities of Russia, it will be considered legitimate.

• Retaliations must be proportionate to the previous attacks (note that previous attacks of Russia included numerous war crimes).
• Retaliations must be aimed only at combatants and military objectives.

References:

Article 51

Nothing in the present Charter shall impair the inherent right of individual or collective self-defence if an armed attack occurs against a Member of the United Nations, until the Security Council has taken measures necessary to maintain international peace and security. Measures taken by Members in the exercise of this right of self-defence shall be immediately reported to the Security Council and shall not in any way affect the authority and responsibility of the Security Council under the present Charter to take at any time such action as it deems necessary in order to maintain or restore international peace and security.

United Nations Charter (full text): https://www.un.org/en/about-us/un-charter/full-text

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In times of conflict, reprisals are considered legal under certain conditions: they must be carried out in response to a previous attack, they must be proportionate to that attack, and they must be aimed only at combatants and military objectives.

Humanitarian law hence forbids all reprisals against civilian persons and objects protected by the 1949 Geneva Conventions and their 1977 Additional Protocols. These include wounded, sick, or shipwrecked persons; medical or religious personnel, units, transports, or material; prisoners of war; civilian persons or civilian objects; cultural property or places of worship; objects indispensable to the survival of the civilian population; the natural environment; works and installations containing dangerous forces; and buildings and material used for the protection of the civilian population (GCI Art. 46; GCII Art. 47; GCIII Art. 13; GCIV Art. 33; API Arts. 20 and 51–56).

Médecins Sans Frontières (Doctors without borders) "The Practical Guide to Humanitarian Law: Reprisals": https://guide-humanitarian-law.org/content/article/3/reprisals/

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A “reprisal” is a breach of international humanitarian law, which would otherwise be unlawful but in exceptional cases is considered lawful as an enforcement measure in response to a previous breach of international humanitarian law by the enemy, with the purpose of terminating the enemy’s violation. Thus, reprisals are intended to put pressure on the enemy in order to obtain the enemy's compliance with international humanitarian law. Reprisals are only allowed under very strict conditions and there is a trend towards outlawing reprisals in international humanitarian law.

Reprisals against wounded, sick or shipwrecked persons, medical or religious personnel, medical units, transports and material, prisoners of war, the civilian population and civilian persons, civilian objects, cultural property, objects indispensable to the survival of the civilian population, the natural environment, works and installations containing dangerous forces and the buildings and material used for the protection of the civilian population are always prohibited.

M. Sassòli, A. Bouvier, A. Quintin, J. Grignon, How does law protect in war?, ICRC, Geneva, 2014: https://casebook.icrc.org/a_to_z/glossary/reprisals