How is a "mercenary soldier" defined in international law?

Question

Sergei Lavrov, the Russian foreign minister has claimed that the two British men, currently under sentence of death, were "mercenaries" attached to the Ukrainian army, and hence not subject to protection under the Geneva convention.

The British case is that the men were regular member of the Ukrainian army, and that such foreigners should be treated in the same way as any other soldiers.

What exactly is it in international law that defines a soldier as a "mercenary"?

Answer 1

Article 47 of the Geneva conventions:

"A mercenary is any person who:

• Is specially recruited locally or abroad in order to fight in an armed conflict;
• Does, in fact, take a direct part in hostilities;
• Is motivated to take part in the hostilities essentially by the desire for private gain and, in fact, is promised, by or on behalf of a Party to the conflict, material compensation substantially in excess of that promised or paid to combatants of similar ranks and functions in the armed forces of that Party;
• Is neither a national of a Party to the conflict nor a resident of territory controlled by a Party to the conflict;
• Is not a member of the armed forces of a Party to the conflict; and
• Has not been sent by a State which is not a Party to the conflict on official duty as a member of its armed forces.

These requirements are cumulative, which means that they must all be applicable for an individual to be categorized as a mercenary."

The UK could certainly claim that the persons involved are/were:

• Not recruited but rather volunteered, therefore the first condition is not met;
• Is not paid "substantially in excess" of ordinary Ukrainian soldiers, so the third condition is not met;
• Is a member of the Ukrainian army, so the fifth condition is not met.

Equally Russia could claim:

• The persons were recruited abroad, or they would not know where to go to once they reached Ukraine;
• The persons were motivated entirely by private gain, in this case the "feel-good" emotion that comes with killing Russians, or possibly they were e.g. unemployed before joining the war and therefore a job as a soldier is still better than starving;
• The persons in question were not Ukrainian soldiers (or did not provide evidence to show they were Ukrainian soldiers), and/or it is allegedly illegal for British citizens to fight for another country against a state that is at peace with the UK anyway.

So, it's going to be a legal wrangle either way.