Let's say that there are 2 countries with very different inhabitants. Country A has no idea that country B exists. Country B may or may not know about Country A. Country A is surrounded by large mountains and has no sea access. Country B is quite far away, on the other side of the mountains. The mountains are only inhabited by wild and dangerous creatures; there is no organized border between the two countries.
The two countries are both the nearly eternal medieval society without gunpowder and with few scientists. There are quite a few other countries on the continent, but none of them has contact with both A and B. Most are in contact with B only.
Around 1000 years ago the two countries (A and B) used to trade and fight together (as allies against a common enemy which they vanquished). Now, however, Country A has forgotten this history and doesn't know what is beyond the mountains.
What properties of my world could lead to Country A losing touch with the rest of the world like that? There is no magic, so no magical spell to make them forget.
How could that situation be possible? Country A not knowing what is on the other side of the mountains?
The main reason I have is that since the trip through the mountains is very difficult, they simply stopped doing it. Being an early medieval civilisation, they did not use much written archives. And in a thousand years they simply forgot the oral memories and it became legends.
I find my explanation very weak and unbelievable. Do you have another explanation for why this loss of common knowledge would happen?