The conflict between the largely Christian (Eastern Orthodox) Serbs and the Muslim Albanians in this region has been going on for about a thousand years. It didn’t start with Hitler and WWII. I know some Serbs who are still bitter about the outcome of the Battle of Kosovo in 1389. Kosovo is claimed by most Serbs to be the ancestral home of Serbia. There is a cauldron of national, ethnic, and religious animosity there that boils over from time to time, aided and abetted by self-interested “allies.”
The change in demographic can be tracked from 1455 (from a Turkish tax census) where about 1/260 of the populace were what we might call "Albanian", and today, when its about 80/100.
I contend that the bulk of this demographic change occurred during WWII, when Hitler and Mussolini tried to establish a Greater Albania. Using genocide.