These 'more encompassing identities' have limited applicability. For example, Polish-, English-, and German-Americans no longer view themselves as separate from each-other, but they still see themselves as separate from African- and Asian-Americans. They've all been in the US for a few centuries now, so it's not limited to a short-term effect.

If you look at other parts of the world, the development of more encompassing identities is anything but guaranteed. From the Bosniaks, Uyghurs, or Roma, groups can live side by side for centuries, without developing joint identities, or developing very fragile ones.

So pretty much the opposite of the identity BS that drives so much of the culture war.

in the long run, there is peace. in the short run ...

In short, short-term pain and long-term gains.

That's not what the study says. It's more: short-term pain and maybe (hopefully) less pain in the long-term.

Besides, I'm not sure how applicable the examples given in the study are to a modern nation state. (You can't run a nation like a military, and religion and nationalism are on the decline, so those are probably not sources of future social cohesion either.)