One of my great-grandfathers was the first partisan in his village, as the Italians occupied his house and then he burned it down with a molotov cocktail.

We're talking about southern Croatia.
His oldest son, my grandfather's brother, was also a partisan, and died - I think he was 17 years old.
The other one was a partisan as well, but I don't know much about him.
Another of my great-grandfathers was an officer in the regular army of the pro-fascist regime, the old gentleman.
Finally, the fourth one was not in a single army because of his principles: the worst scumbags from his village went to armies of both of the warring sides*, and he didn't want to go to either of those armies when they accepted such bastards

I don't know how he avoided the draft into the regular Home Guard, well, I think there was no general mobilisation.
*The right-wing Home Guard (regular army of the Independent State of Croatia, named so for historical reasons, it wasn't some gun club but a real army) and the nazi Ustashe (simplified, the Croatian fascist party, and they had the Ustashe Militia which was akin to Waffen SS and was to the Home Guard what Waffen SS was to the Heer), and the left-wing Partisans, who also accepted apolitical or democratic members, but after '42 most of the commanders were communists.
WW2 in ex-Yugoslavia was very bloody and tragic. Not only did all of the people of these 6 neighbouring nations kill each others, they killed themselves amongst themselves as well. Most of Croatia and Bosnia-Herzegowina were a part of the fascist Croatia, which fought versus pan-Yugoslavian partisans, which included many Croats. Plus the Yugoslav royalists, Serbian chetniks, on Croatian territory. In Serbia, Germans vs. partisans vs. chetniks.
Also, the war is largely ignored, whereas from a military history afficionado's point of view, it is really interesting and large-scale.
The Yugoslav partisans, unlike the French, Norwegian, Greek etc. did not fight only small scale battles and raids: they actually fought large-scale pitched battles - like the Battle of Neretva river, with the movie which was nominated for an Oscar. There were sabotages, raids and espionage, of course, but there was real warfare from '42 on.
Also, the Yugoslav partisans are the only ones which actually liberated their country on their own (they received some armament, but mostly took it from their enemies, and even manufactured their own, like the Polish.
They maintained ''liberated territories'' complete with civilian administration at all times. We're talking about towns and regions, not a couple of villages.
Numbers wise: the Yugoslavian Army, which was really a true army, which grew from the partisans, had 800 000 soldiers in '45. Yes, eight hundred thousand. Complete with divisions, corpses, armies.
The Croatian Army had 100 000 - 200 000 at various points. 18 divisions in 1945. They also had a fairly good airforce for the size and significance of the country, they even received 100+ Messerschmitt 109's. Compared to our 10 MiG 21's today, they had a better airforce

The Serb royalist chetniks also had about 150 000 across whole ex-Yugoslavia.
EDIT: To further illustrate the tragic nature of the war, now I remembered, my great grandmother's 3 brothers, in Northern Croatia, were a Home Guardsman (Croatian Heer), an Ustasha (Croatian Waffen-SS), and a partisan.
Truly a fratricidal war.