Not a real argument, but I don't think I can come up with a real argument for your case, so fair enough.
Show me a single case where previously-warring nations peacefully unified (i.e. not through conquest or subjugation)? Poland-Lithuania and England-Scotland are the only two I can think of.
Because the counterexample--multiethnic nations that split along national-identity lines--is far more frequent since the age of conquest. Former Yugoslavia. Pre-Partition India. Sudan. Ethiopia. Algeria.
Multiethnicisim is hard. Where it works, it happened through immigration. Combining previously-warring nations under one roof is basically just assisted civil war.
