Nationalism is comprised of a set of concepts, including
- the idea that every human being "naturally" belongs to a nation,
- the idea that a nation has a territory which it "rightfully" occupies,
- some idea that nations are ethical subjects in some way, entitled to self determination (where self-determination of the nation is sometimes detached from self-determination of the humans that constitute the nation)
Determining which land belongs to what nation is entirely impossible. After millennia of history involving countless events of migration, occuptation, naturalisation, displacement and so forth, almost any patch of land can be claimed by more than one nation.
The idea to promote or enable peace by enabling a nation to determine its own fate is therefore very problematic. Peace is a condition that enables human (i.e. individual) self-determination, and care should be taken not to conflate that with the (largely artificial) notion of national self-determination.