Russia and Ukraine have mobilised hundreds of thousands of soldiers to fight the war.
Both countries have also made calls for foreigners to join their side.
Ukraine says up to 20,000 volunteers from 52 countries have signed up to join an international legion.
On the Russian side, President Vladimir Putin has approved allowing foreign fighters, with the defence ministry saying 16,000 volunteers from the Middle East are ready to join.
The Kremlin says those from the West who fight for Ukraine will be considered mercenaries, with no protection under the Geneva Convention.
So, what consequences will foreign fighters face under international law?
And how will they affect the conflict?
Presenter: Mohammed Jamjoom
Pavel Felgenhauer – Defence and military analyst
Anthony Dworkin – Senior policy fellow, European Council on Foreign Relations
Omar Ashour – Founding chair, Doha Institute’s Critical Security Studies programme