Question

Why would some people and nations oppose trade and specialization?

Answer 1

My thoughts on the matter are that the people who oppose trade and specialization don't want foreign influences getting involved with their business, which would be what China and Japan pulled off in their history (the former had severe issues with Western Powers showing up, which led to various conflicts including The Boxer Rebellion, and the latter more or less isolated themselves for roughly a thousand years or something like that).

There are other reasons, but that's the one I thought of first. Hope it helps.