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.
Not many with that sort of history. It's barely working in the EU, and that's because America took away the toys for a while.