Complex sentence: Most are subnational regions with an ethnic or national identity of their own that have "broken off" (i.e., separated themselves) from their original parent state, and hence they are commonly referred to as "break-away" states.
Split into simpler sentences: Most are subnational regions with an ethnic or national identity of their own that have separated themselves from their original parent state. Such states are commonly referred to as "break-away" states.