To be explanatory, I ruled the country of Egypt in the "European Clash of Nations" event and came as far as the Turkish border.
I put 3 different soldiers at a level very close to the Turkish border for all the Turkey entrances of the walking routes in the Homs, Aleppo and Shengal rural areas.
ı mean, no way to they can enter to my land.
Shengal provincial morale dropped drastically and it must have joined Turkey because I annexed Syria completely.
Could there be a logical explanation for this

Actually yes, that land was flashing red but ı did not at war with Turkey, just ı was waiting near the borders, so not war

is that means, even not their country, not my enemy or not their ally; they will be join wherever close to them instead of me