What happens if a country commits a war crime?

With the conflict between Russia and Ukraine carrying on, the eyes of the world are fixed on invading troops to ensure no war crimes are committed.

Russia's invasion has essentially broken nearly 100 years of international norms in Europe to not pursue territorial conquest, but does it inflict upon international law?

While it used to be common to see invasions during the 20th century and before, World War II changed a lot of norms and behaviours of countries in Europe which deterred neighbouring countries from carrying out invasions.