Reading the GP made me immediately think of India as a counterexample, nice to see someone else did too. :)
India was never a "country" before the Brits, it was just a geographic region with some commonalities. Various empires have covered large swathes of India, but there wasn't any common identity. By being lumped into a single country by the British, India got this identity.
India has many problems but "regionalism" isn't one of them. It exists and causes issues every now and then, but not too much.
However, India has had a catalyst here as you mention. The first was having little choice in forming this union[1], and the second was the "common enemy" of the British bonding otherwise-distinct groups of people together.
Ironically, the country most Indians hate the most is Pakistan, despite it having been part of the "common identity" and having the same catalysts. Part of this is deliberate divide-and-conquer (divide-and-cripple, really, this was done after the country was conquered; during the independence process) by the British, part of it is simple religious hatred. Though India has a much nicer relationship with, say, Afghanistan (but some of the reasons for that involve Pakistan ... so it's nuanced).
So if India had never been a single nation and had to form an AU- or EU-like union, its quite possible that the relations between various regions would have been much more strained -- India-Pakistan relations are an example of how it could have gone. This is all speculation and Africa is very different anyway, so I'm not quite sure if this will be a problem there. I'm optimistic.
[1]: Though many leaders from the revolutionary movement have a hand in avoiding the fragmentation of some parts of India, which was on the table. But the common "Indian" identity existed before independence, due to being part of the same colony.
India was never a "country" before the Brits, it was just a geographic region with some commonalities. Various empires have covered large swathes of India, but there wasn't any common identity. By being lumped into a single country by the British, India got this identity.
India has many problems but "regionalism" isn't one of them. It exists and causes issues every now and then, but not too much.
However, India has had a catalyst here as you mention. The first was having little choice in forming this union[1], and the second was the "common enemy" of the British bonding otherwise-distinct groups of people together.
Ironically, the country most Indians hate the most is Pakistan, despite it having been part of the "common identity" and having the same catalysts. Part of this is deliberate divide-and-conquer (divide-and-cripple, really, this was done after the country was conquered; during the independence process) by the British, part of it is simple religious hatred. Though India has a much nicer relationship with, say, Afghanistan (but some of the reasons for that involve Pakistan ... so it's nuanced).
So if India had never been a single nation and had to form an AU- or EU-like union, its quite possible that the relations between various regions would have been much more strained -- India-Pakistan relations are an example of how it could have gone. This is all speculation and Africa is very different anyway, so I'm not quite sure if this will be a problem there. I'm optimistic.
[1]: Though many leaders from the revolutionary movement have a hand in avoiding the fragmentation of some parts of India, which was on the table. But the common "Indian" identity existed before independence, due to being part of the same colony.