When does a group of people have the right to become a nation?

They have the right to become a nation when their real nation doesn't follow their part. Even thought the real nation might not treat them as their own nation. They still have the right to become their own nation when lets say that the real nation's government says if the people does something then they'll do some thing in return and people so their part but the government doesn't follow their part the people have the right to become their nation.