Complex sentence: A nation is a cultural entity, and not necessarily a political association, nor is it necessarily linked to a particular territorial area--although nationalists argue that the boundaries of a nation and a state should, as far as possible, coincide.
Split into simpler sentences: A nation is best viewed as a cultural entity and not a political association; nor is it necessarily linked to a particular territorial area. But nationalists hold the opposite as self-evident: that the boundaries of a nation and a state should, as far as possible, coincide.