King Bowser's kidnapping of Princess Toadstool is usually treated as a flagrant breach of international law, yet evidence points to the Koopa minority in the Mushroom Kingdom being regularly discriminated against and subject to mistreatment at the hand of the plumber population. Was Koopa intervention justified in such circumstances?
What 'evidence'? The Koopas of Koopa Village make no mention of discrimination or border conflicts, despite being near Peach's Castle.
You'll have to be specific. Princess Peach was kidnapped no less than on ten separate occasions. In general, the forced abduction of another country's leader, especially without a declaration of war would be considered a violation of international law. Regardless of this, without being specific, I'm afraid I can't be of much help to you.
On another note, please don't call her "Toadstool". That is an outdated translation and is considered borderline offensive to the people of the Mushroom Kingdom.