Russia’s war on Ukraine shows you can’t stop a war in another country even with massive sanctions.

And, various empires’ adventures in Afghanistan show that even sending in troops to stop the “bad guys” will probably not work.

Sometimes people can make a change. The Apartheid governments of South Africa collapsed largely due to boycotts and people pressure. But, that only really works when the targeted country wants to be part of the community of nations, and its people don’t want to be seen as criminals by the rest of the world. IMO, that situation is rare. Most of the time the rest of the world can’t do much of anything when it comes to civil wars and border conflicts.